Indeed, the Gtk+ and WPE ports, as well as Sony's Windows port, are the most active after the Apple-maintained ones.

Sony's Windows Port?

Thanks to these comments today I am finally running WebKit on Windows for preliminary Safari compatibility testing purposes.

This blog[1] explains how to get Windows builds[2] from the WebKit Build Archives on S3 that require Apple Application Support[3].

[1] https://medium.com/@alSkachkov/how-to-load-the-latest-webkit...

[2] https://build.webkit.org/builders/Apple%20Win%2010%20Release...

[3] 7-zip the iTunes installer to extract & install just AppleApplicationSupport.msi https://www.apple.com/itunes/download/win64

They maintain a Windows port of WebKit as a reference implementation.

the browser in Steam client also uses webkit

No, it does not. Steam has been using Chromium Embedded Framework since 2010, so when Chromium moved to Blink (~2013), so did Steam.