YoutubeHideWatched.user.js

// ==UserScript==
// @version        1.4.1
// @name           Hide watched videos on YouTube
// @icon           https://www.youtube.com/favicon.ico
// @match          https://www.youtube.com/*
// @exclude        https://www.youtube.com/embed/*
// @exclude        https://www.youtube.com/api/*
// @namespace      https://gist.github.com/xPaw/6324624
// @updateURL      https://gist.github.com/xPaw/6324624/raw/YoutubeHideWatched.user.js
// @downloadURL    https://gist.github.com/xPaw/6324624/raw/YoutubeHideWatched.user.js
// @grant          none
// ==/UserScript==

const app = document.querySelector( 'ytd-app' );

function HideVideos( a )
{
	app.querySelectorAll( 'ytd-browse[page-subtype="subscriptions"] ytd-thumbnail-overlay-resume-playback-renderer' ).forEach( element =>
	{
		// Find the container element for this video
		let tagName;

		do
		{
			element = element.parentNode;
			tagName = element.tagName.toLowerCase();
		}
		while( tagName !== 'ytd-grid-video-renderer' && tagName !== 'ytd-item-section-renderer' );

		element.hidden = true;
	} );
}

function ProcessPage()
{
	if( !window.location.pathname.startsWith( '/feed/subscriptions' ) )
	{
		return;
	}

	const list = app.querySelector( 'ytd-section-list-renderer' );

	if( list.dataset.hooked )
	{
		return;
	}

	list.dataset.hooked = true;
	list.addEventListener( 'yt-next-continuation-data-updated', HideVideos );

	// TODO: Find an event to fix this
	new MutationObserver( HideVideos ).observe( list, { childList: true, subtree: true } );
}

if( app )
{
    app.addEventListener( 'yt-navigate-finish', ProcessPage );
}

ProcessPage();

Works now.
Thank you very much ;)

For those wondering, this script doesn't hide video permanently. If you reload, all videos will be back there.

It stops working for me/doesn't hide watched videos anymore, maybe YouTube change something? Could you please check/update the script? Thanks!

Not working on the front page at the moment version 1.4.0

Not working on the front page at the moment version 1.4.0

In order to make it work on homepage we need to include a new tagName to the filtering list:

change:
while( tagName !== 'ytd-grid-video-renderer' && tagName !== 'ytd-item-section-renderer' );

to:
while( tagName !== 'ytd-grid-video-renderer' && tagName !== 'ytd-item-section-renderer' && tagName !== 'ytd-rich-item-renderer' );

or get it at my fork https://gist.github.com/frbaroni/7652d14f265ef67e37652d25206369ab

Not working on the front page at the moment version 1.4.0

In order to make it work on homepage we need to include a new tagName to the filtering list:

change:
while( tagName !== 'ytd-grid-video-renderer' && tagName !== 'ytd-item-section-renderer' );

to:
while( tagName !== 'ytd-grid-video-renderer' && tagName !== 'ytd-item-section-renderer' && tagName !== 'ytd-rich-item-renderer' );

or get it at my fork https://gist.github.com/frbaroni/7652d14f265ef67e37652d25206369ab

Thanks!