PowerShell encoding Zip paths to use forward slash (Zip Spec) instead of backslash (Windows Style); for portable zip files - thanks to @sethjackson

portable-zip-pathnames.ps1

# When using System.IO.Compression.ZipFile.CreateFromDirectory in PowerShell, it still uses backslashes in the zip paths
# despite this https://docs.microsoft.com/en-us/dotnet/framework/migration-guide/mitigation-ziparchiveentry-fullname-path-separator

# Based upon post by Seth Jackson https://sethjackson.github.io/2016/12/17/path-separators/

#
# PowerShell 5 (WMF5) & 6
# Using class Keyword https://msdn.microsoft.com/powershell/reference/5.1/Microsoft.PowerShell.Core/about/about_Classes
#

Add-Type -AssemblyName System.Text.Encoding
Add-Type -AssemblyName System.IO.Compression.FileSystem

class FixedEncoder : System.Text.UTF8Encoding {
    FixedEncoder() : base($true) { }

    [byte[]] GetBytes([string] $s)
    {
        $s = $s.Replace("\\", "/");
        return ([System.Text.UTF8Encoding]$this).GetBytes($s);
    }
}

[System.IO.Compression.ZipFile]::CreateFromDirectory($dirToZip, $zipFilePath, [System.IO.Compression.CompressionLevel]::Optimal, $false, [FixedEncoder]::new())

#
# PowerShell 4 (WMF4)
# Using Add-Type cmdlet https://msdn.microsoft.com/en-us/powershell/reference/4.0/microsoft.powershell.utility/add-type
#

Add-Type -AssemblyName System.Text.Encoding
Add-Type -AssemblyName System.IO.Compression.FileSystem

$EncoderClass=@"
  public class FixedEncoder : System.Text.UTF8Encoding {
    public FixedEncoder() : base(true) { }

    public override byte[] GetBytes(string s) {
      s = s.Replace("\\", "/");
      return base.GetBytes(s);
    }
  }
"@
Add-Type -TypeDefinition $EncoderClass

$Encoder = New-Object FixedEncoder
[System.IO.Compression.ZipFile]::CreateFromDirectory($dirToZip, $zipFilePath, [System.IO.Compression.CompressionLevel]::Optimal, $false, $Encoder)

In order for these to work this line needs to be replaced:

s = s.Replace("\\", "/");

with

s = s.Replace("\", "/");

There is no need to escape the \ character here. Presumably that was the intention of the double backslash.

I made this modification and was able to ZIP a directory on Windows Server 2016 and expand the ZIP on Mac OSX 10.13.5.