Modified VLC youtube.lua using youtube-dl. The latest youtube.lua doesn't load all videos resulting in a 403 error. Youtube-dl manages to get the direct link however many videos which previously failed to load using the original script do not play in high resolution for reasons probably related to changes in Youtube itself.

youtube.lua

--[[
 $Id$

 Copyright © 2007-2020 the VideoLAN team

 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation; either version 2 of the License, or
 (at your option) any later version.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with this program; if not, write to the Free Software
 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
--]]

-- Helper function to get a parameter's value in a URL
function get_url_param( url, name )
    local _, _, res = string.find( url, "[&?]"..name.."=([^&]*)" )
    return res
end

-- Helper function to copy a parameter when building a new URL
function copy_url_param( url, name )
    local value = get_url_param( url, name )
    return ( value and "&"..name.."="..value or "" ) -- Ternary operator
end

function get_arturl()
    local iurl = get_url_param( vlc.path, "iurl" )
    if iurl then
        return iurl
    end
    local video_id = get_url_param( vlc.path, "v" )
    if not video_id then
        return nil
    end
    return vlc.access.."://img.youtube.com/vi/"..video_id.."/default.jpg"
end

-- Pick the most suited format available
function get_fmt( fmt_list )
    local prefres = vlc.var.inherit(nil, "preferred-resolution")
    if prefres < 0 then
        return nil
    end

    local fmt = nil
    for itag,height in string.gmatch( fmt_list, "(%d+)/%d+x(%d+)[^,]*" ) do
        -- Apparently formats are listed in quality
        -- order, so we take the first one that works,
        -- or fallback to the lowest quality
        fmt = itag
        if tonumber(height) <= prefres then
            break
        end
    end
    return fmt
end

-- Helper emulating vlc.readline() to work around its failure on
-- very long lines (see #24957)
function read_long_line()
    local eol
    local pos = 0
    local len = 32768
    repeat
        len = len * 2
        local line = vlc.peek( len )
        if not line then return nil end
        eol = string.find( line, "\n", pos + 1 )
        pos = len
    until eol or len >= 1024 * 1024 -- No EOF detection, loop until limit
    return vlc.read( eol or len )
end

-- Buffering iterator to parse through the HTTP stream several times
-- without making several HTTP requests
function buf_iter( s )
    s.i = s.i + 1
    local line = s.lines[s.i]
    if not line then
        -- Put back together statements split across several lines,
        -- otherwise we won't be able to parse them
        repeat
            local l = s.stream:readline()
            if not l then break end
            line = line and line..l or l -- Ternary operator
        until string.match( line, "};$" )

        if line then
            s.lines[s.i] = line
        end
    end
    return line
end

-- Helper to search and extract code from javascript stream
function js_extract( js, pattern )
    js.i = 0 -- Reset to beginning
    for line in buf_iter, js do
        local ex = string.match( line, pattern )
        if ex then
            return ex
        end
    end
    vlc.msg.err( "Couldn't process youtube video URL, please check for updates to this script" )
    return nil
end

-- Descramble the URL signature using the javascript code that does that
-- in the web page
function js_descramble( sig, js_url )
    -- Fetch javascript code
    local js = { stream = vlc.stream( js_url ), lines = {}, i = 0 }
    if not js.stream then
        vlc.msg.err( "Couldn't process youtube video URL, please check for updates to this script" )
        return sig
    end

    -- Look for the descrambler function's name
    -- if(k.s){var l=k.sp,m=pt(decodeURIComponent(k.s));f.set(l,encodeURIComponent(m))}
    -- k.s (from stream map field "s") holds the input scrambled signature
    -- k.sp (from stream map field "sp") holds a parameter name (normally
    -- "signature" or "sig") to set with the output, descrambled signature
    local descrambler = js_extract( js, "[=%(,&|](..)%(decodeURIComponent%(.%.s%)%)" )
    if not descrambler then
        vlc.msg.dbg( "Couldn't extract youtube video URL signature descrambling function name" )
        return sig
    end

    -- Fetch the code of the descrambler function
    -- Go=function(a){a=a.split("");Fo.sH(a,2);Fo.TU(a,28);Fo.TU(a,44);Fo.TU(a,26);Fo.TU(a,40);Fo.TU(a,64);Fo.TR(a,26);Fo.sH(a,1);return a.join("")};
    local rules = js_extract( js, "^"..descrambler.."=function%([^)]*%){(.-)};" )
    if not rules then
        vlc.msg.dbg( "Couldn't extract youtube video URL signature descrambling rules" )
        return sig
    end

    -- Get the name of the helper object providing transformation definitions
    local helper = string.match( rules, ";(..)%...%(" )
    if not helper then
        vlc.msg.dbg( "Couldn't extract youtube video URL signature transformation helper name" )
        vlc.msg.err( "Couldn't process youtube video URL, please check for updates to this script" )
        return sig
    end

    -- Fetch the helper object code
    -- var Fo={TR:function(a){a.reverse()},TU:function(a,b){var c=a[0];a[0]=a[b%a.length];a[b]=c},sH:function(a,b){a.splice(0,b)}};
    local transformations = js_extract( js, "[ ,]"..helper.."={(.-)};" )
    if not transformations then
        vlc.msg.dbg( "Couldn't extract youtube video URL signature transformation code" )
        return sig
    end

    -- Parse the helper object to map available transformations
    local trans = {}
    for meth,code in string.gmatch( transformations, "(..):function%([^)]*%){([^}]*)}" ) do
        -- a=a.reverse()
        if string.match( code, "%.reverse%(" ) then
          trans[meth] = "reverse"

        -- a.splice(0,b)
        elseif string.match( code, "%.splice%(") then
          trans[meth] = "slice"

        -- var c=a[0];a[0]=a[b%a.length];a[b]=c
        elseif string.match( code, "var c=" ) then
          trans[meth] = "swap"
        else
            vlc.msg.warn("Couldn't parse unknown youtube video URL signature transformation")
        end
    end

    -- Parse descrambling rules, map them to known transformations
    -- and apply them on the signature
    local missing = false
    for meth,idx in string.gmatch( rules, "..%.(..)%([^,]+,(%d+)%)" ) do
        idx = tonumber( idx )

        if trans[meth] == "reverse" then
            sig = string.reverse( sig )

        elseif trans[meth] == "slice" then
            sig = string.sub( sig, idx + 1 )

        elseif trans[meth] == "swap" then
            if idx > 1 then
                sig = string.gsub( sig, "^(.)("..string.rep( ".", idx - 1 )..")(.)(.*)$", "%3%2%1%4" )
            elseif idx == 1 then
                sig = string.gsub( sig, "^(.)(.)", "%2%1" )
            end
        else
            vlc.msg.dbg("Couldn't apply unknown youtube video URL signature transformation")
            missing = true
        end
    end
    if missing then
        vlc.msg.err( "Couldn't process youtube video URL, please check for updates to this script" )
    end
    return sig
end

-- Parse and assemble video stream URL
function stream_url( params, js_url )
    local url = string.match( params, "url=([^&]+)" )
    if not url then
        return nil
    end
    url = vlc.strings.decode_uri( url )

    -- Descramble any scrambled signature and append it to URL
    local s = string.match( params, "s=([^&]+)" )
    if s then
        s = vlc.strings.decode_uri( s )
        vlc.msg.dbg( "Found "..string.len( s ).."-character scrambled signature for youtube video URL, attempting to descramble... " )
        if js_url then
            s = js_descramble( s, js_url )
        else
            vlc.msg.err( "Couldn't process youtube video URL, please check for updates to this script" )
        end

        local sp = string.match( params, "sp=([^&]+)" )
        if not sp then
            vlc.msg.warn( "Couldn't extract signature parameters for youtube video URL, guessing" )
            sp = "signature"
        end
        url = url.."&"..sp.."="..vlc.strings.encode_uri_component( s )
    end

    return url
end

-- Parse and pick our video stream URL (classic parameters)
function pick_url( url_map, fmt, js_url )
    for stream in string.gmatch( url_map, "[^,]+" ) do
        local itag = string.match( stream, "itag=(%d+)" )
        if not fmt or not itag or tonumber( itag ) == tonumber( fmt ) then
            return stream_url( stream, js_url )
        end
    end
    return nil
end

-- Parse and pick our video stream URL (new-style parameters)
function pick_stream( stream_map, js_url )
    local pick = nil

    local fmt = tonumber( get_url_param( vlc.path, "fmt" ) )
    if fmt then
        -- Legacy match from URL parameter
        for stream in string.gmatch( stream_map, '{(.-)}' ) do
            local itag = tonumber( string.match( stream, '"itag":(%d+)' ) )
            if fmt == itag then
                pick = stream
                break
            end
        end
    else
        -- Compare the different available formats listed with our
        -- quality targets
        local prefres = vlc.var.inherit( nil, "preferred-resolution" )
        local bestres = nil

        for stream in string.gmatch( stream_map, '{(.-)}' ) do
            local height = tonumber( string.match( stream, '"height":(%d+)' ) )

            -- Better than nothing
            if not pick or ( height and ( not bestres
                -- Better quality within limits
                or ( ( prefres < 0 or height <= prefres ) and height > bestres )
                -- Lower quality more suited to limits
                or ( prefres > -1 and bestres > prefres and height < bestres )
            ) ) then
                bestres = height
                pick = stream
            end
        end
    end

    if not pick then
        return nil
    end

    -- Either the "url" or the "signatureCipher" parameter is present,
    -- depending on whether the URL signature is scrambled.
    local cipher = string.match( pick, '"signatureCipher":"(.-)"' )
        or string.match( pick, '"[a-zA-Z]*[Cc]ipher":"(.-)"' )
    if cipher then
        -- Scrambled signature: some assembly required
        local url = stream_url( cipher, js_url )
        if url then
            return url
        end
    end
    -- Unscrambled signature, already included in ready-to-use URL
    return string.match( pick, '"url":"(.-)"' )
end

-- Probe function.
function probe()
    return ( ( vlc.access == "http" or vlc.access == "https" ) and (
            ((
               string.match( vlc.path, "^www%.youtube%.com/" )
            or string.match( vlc.path, "^music%.youtube%.com/" )
            or string.match( vlc.path, "^gaming%.youtube%.com/" ) -- out of use
             ) and (
               string.match( vlc.path, "/watch%?" ) -- the html page
            or string.match( vlc.path, "/live$" ) -- user live stream html page
            or string.match( vlc.path, "/live%?" ) -- user live stream html page
            or string.match( vlc.path, "/get_video_info%?" ) -- info API
            or string.match( vlc.path, "/v/" ) -- video in swf player
            or string.match( vlc.path, "/embed/" ) -- embedded player iframe
             )) or
               string.match( vlc.path, "^consent%.youtube%.com/" )
         ) )
end

-- Parse function.
function parse()
    if string.match( vlc.path, "^consent%.youtube%.com/" ) then
        -- Cookie consent redirection
        -- Location: https://consent.youtube.com/m?continue=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DXXXXXXXXXXX&gl=FR&m=0&pc=yt&uxe=23983172&hl=fr&src=1
        -- Set-Cookie: CONSENT=PENDING+355; expires=Fri, 01-Jan-2038 00:00:00 GMT; path=/; domain=.youtube.com
        local url = get_url_param( vlc.path, "continue" )
        if not url then
            vlc.msg.err( "Couldn't handle YouTube cookie consent redirection, please check for updates to this script or try disabling HTTP cookie forwarding" )
            return { }
        end
        return { { path = vlc.strings.decode_uri( url ), options = { ":no-http-forward-cookies" } } }
    elseif not string.match( vlc.path, "^www%.youtube%.com/" ) then
        -- Skin subdomain
        return { { path = vlc.access.."://"..string.gsub( vlc.path, "^([^/]*)/", "www.youtube.com/" ) } }

    elseif string.match( vlc.path, "/watch%?" )
        or string.match( vlc.path, "/live$" )
        or string.match( vlc.path, "/live%?" )
    then -- This is the HTML page's URL
        local js_url
        -- fmt is the format of the video
        -- (cf. http://en.wikipedia.org/wiki/YouTube#Quality_and_formats)
        fmt = get_url_param( vlc.path, "fmt" )
        while true do
            -- The new HTML code layout has fewer and longer lines; always
            -- use the long line workaround until we get more visibility.
            local line = new_layout and read_long_line() or vlc.readline()
            if not line then break end

            -- The next line is the major configuration line that we need.
            -- It is very long so we need this workaround (see #24957).
            if string.match( line, '^ *<div id="player%-api">' ) then
                line = read_long_line()
                if not line then break end
            end

            if not title then
                local meta = string.match( line, '<meta property="og:title"( .-)>' )
                if meta then
                    title = string.match( meta, ' content="(.-)"' )
                    if title then
                        title = vlc.strings.resolve_xml_special_chars( title )
                    end
                end
            end

            if not description then
                -- FIXME: there is another version of this available,
                -- without the double JSON string encoding, but we're
                -- unlikely to access it due to #24957
                description = string.match( line, '\\"shortDescription\\":\\"(.-[^\\])\\"')
                if description then
                    -- FIXME: do this properly (see #24958)
                    description = string.gsub( description, '\\(["\\/])', '%1' )
                else
                    description = string.match( line, '"shortDescription":"(.-[^\\])"')
                end
                if description then
                    if string.match( description, '^"' ) then
                        description = ""
                    end
                    -- FIXME: do this properly (see #24958)
                    -- This way of unescaping is technically wrong
                    -- so as little as possible of it should be done
                    description = string.gsub( description, '\\(["\\/])', '%1' )
                    description = string.gsub( description, '\\n', '\n' )
                    description = string.gsub( description, '\\r', '\r' )
                    description = string.gsub( description, "\\u0026", "&" )
                end
            end

            if not arturl then
                local meta = string.match( line, '<meta property="og:image"( .-)>' )
                if meta then
                    arturl = string.match( meta, ' content="(.-)"' )
                    if arturl then
                        arturl = vlc.strings.resolve_xml_special_chars( arturl )
                    end
                end
            end

            if not artist then
                artist = string.match(line, '\\"author\\":\\"(.-)\\"')
                if artist then
                    -- FIXME: do this properly (see #24958)
                    artist = string.gsub( artist, '\\(["\\/])', '%1' )
                else
                    artist = string.match( line, '"author":"(.-)"' )
                end
                if artist then
                    -- FIXME: do this properly (see #24958)
                    artist = string.gsub( artist, "\\u0026", "&" )
                end
            end

            if not new_layout then
                if string.match( line, '<script nonce="' ) then
                    vlc.msg.dbg( "Detected new YouTube HTML code layout" )
                    new_layout = true
                end
            end
        end

        if not path then
            local cmd = "youtube-dl.exe -f best --get-url "..vlc.access.."://"..vlc.path
            local f = io.popen (cmd, 'r')
            local output = f:read('*all')
            f:close()
            path = output:sub(1, -2)
        end

        if not path then
            vlc.msg.err( "Couldn't extract youtube video URL, please check for updates to this script" )
            return { }
        end

        if not arturl then
            arturl = get_arturl()
        end

        return { { path = path; name = title; description = description; artist = artist; arturl = arturl } }

    elseif string.match( vlc.path, "/get_video_info%?" ) then -- video info API
        local line = vlc.read( 1024*1024 ) -- data is on one line only
        if not line then
            vlc.msg.err( "YouTube API output missing" )
            return { }
        end

        local js_url = get_url_param( vlc.path, "jsurl" )
        if js_url then
            js_url= vlc.strings.decode_uri( js_url )
        end

        -- Classic parameters - out of use since early 2020
        local fmt = get_url_param( vlc.path, "fmt" )
        if not fmt then
            local fmt_list = string.match( line, "&fmt_list=([^&]*)" )
            if fmt_list then
                fmt_list = vlc.strings.decode_uri( fmt_list )
                fmt = get_fmt( fmt_list )
            end
        end

        local url_map = string.match( line, "&url_encoded_fmt_stream_map=([^&]*)" )
        if url_map then
            vlc.msg.dbg( "Found classic parameters for youtube video stream, parsing..." )
            url_map = vlc.strings.decode_uri( url_map )
            path = pick_url( url_map, fmt, js_url )
        end

        -- New-style parameters
        if not path then
            local stream_map = string.match( line, '%%22formats%%22%%3A%%5B(.-)%%5D' )
            if stream_map then
                vlc.msg.dbg( "Found new-style parameters for youtube video stream, parsing..." )
                stream_map = vlc.strings.decode_uri( stream_map )
                -- FIXME: do this properly (see #24958)
                stream_map = string.gsub( stream_map, "\\u0026", "&" )
                path = pick_stream( stream_map, js_url )
            end
        end

        if not path then
            -- If this is a live stream, the URL map will be empty
            -- and we get the URL from this field instead
            local hlsvp = string.match( line, "%%22hlsManifestUrl%%22%%3A%%22(.-)%%22" )
            if hlsvp then
                hlsvp = vlc.strings.decode_uri( hlsvp )
                path = hlsvp
            end
        end

        if not path and get_url_param( vlc.path, "el" ) ~= "detailpage" then
            -- Retry with the other known value for the "el" parameter;
            -- either value has historically been wrong and failed for
            -- some videos but not others.
            local video_id = get_url_param( vlc.path, "video_id" )
            if video_id then
                path = vlc.access.."://www.youtube.com/get_video_info?video_id="..video_id.."&el=detailpage"..copy_url_param( vlc.path, "fmt" )..copy_url_param( vlc.path, "jsurl" )
                vlc.msg.warn( "Couldn't extract video URL, retrying with alternate YouTube API parameters" )
            end
        end

        if not path then
            vlc.msg.err( "Couldn't extract youtube video URL, please check for updates to this script" )
            return { }
        end

        local title = string.match( line, "%%22title%%22%%3A%%22(.-)%%22" )
        if title then
            title = string.gsub( title, "+", " " )
            title = vlc.strings.decode_uri( title )
            -- FIXME: do this properly (see #24958)
            title = string.gsub( title, "\\u0026", "&" )
        end
        -- FIXME: description gets truncated if it contains a double quote
        local description = string.match( line, "%%22shortDescription%%22%%3A%%22(.-)%%22" )
        if description then
            description = string.gsub( description, "+", " " )
            description = vlc.strings.decode_uri( description )
            -- FIXME: do this properly (see #24958)
            description = string.gsub( description, '\\(["\\/])', '%1' )
            description = string.gsub( description, '\\n', '\n' )
            description = string.gsub( description, '\\r', '\r' )
            description = string.gsub( description, "\\u0026", "&" )
        end
        local artist = string.match( line, "%%22author%%22%%3A%%22(.-)%%22" )
        if artist then
            artist = string.gsub( artist, "+", " " )
            artist = vlc.strings.decode_uri( artist )
            -- FIXME: do this properly (see #24958)
            artist = string.gsub( artist, "\\u0026", "&" )
        end
        local arturl = string.match( line, "%%22playerMicroformatRenderer%%22%%3A%%7B%%22thumbnail%%22%%3A%%7B%%22thumbnails%%22%%3A%%5B%%7B%%22url%%22%%3A%%22(.-)%%22" )
        if arturl then
            arturl = vlc.strings.decode_uri( arturl )
        end

        return { { path = path, name = title, description = description, artist = artist, arturl = arturl } }

    else -- Other supported URL formats
        local video_id = string.match( vlc.path, "/[^/]+/([^?]*)" )
        if not video_id then
            vlc.msg.err( "Couldn't extract youtube video URL" )
            return { }
        end
        return { { path = vlc.access.."://www.youtube.com/watch?v="..video_id..copy_url_param( vlc.path, "fmt" ) } }
    end
end

I still get 403 responses using this.