The webkitgtk-6.0 API version (providing a GTK 4 API) is now stable as of WebKitGTK 2.39.91. Go ahead and start using it!
The previous webkit2gtk-5.0 API version is already removed. Very few applications used it, and I’ve already worked with them to update to webkitgtk-6.0.
Currently the documentation is not yet available online, but most Linux distros will have doc packages available. A migration guide is available too; if you don’t have easy access to a distro docs package, you can view the markdown source.
Oh, one more thing: WebKitGTK 2.39.91 will be made available in the GNOME master and 44 runtimes shortly.
When WebKitGTK 2.40.0 is released to the GNOME 43 runtime, it will be released for GTK 3 only. We will not provide webkitgtk-6.0 in the 43 runtime and will continue to use WebKitGTK 2.38 for GTK 4. This means there will be no further security updates for affected applications using that runtime. I don’t think this will be a problem in practice; it will impact Builder, but Builder should quickly move to the 44 runtime, and does not load much untrusted content anyway.
That’s impractical. There are dozens of apps using WebKitGTK and they can’t all be migrated to GTK 4 within 6 months. We’ve now got GNOME down to just webkit2gtk-4.1 and webkitgtk-6.0, but all major distros still need webkit2gtk-4.0 for now. I’m going to try tackling that in Fedora soon.