WebKit went multiprocess with the release of WebKit2 around 14 years ago, with the difference being that the multiprocess architecture is part of WebKit itself and thus easily reusable — just embed a WebView in your app and you have it. This contrasts to the Chromium implementation where multiprocess is handled by Chromium rather than Blink, meaning to get multiprocess you have to ship the whole of Chromium and can’t just embed Blink.

That said this really only relevant for Apple platforms and Linux/Android, unfortunately. WebKit for Windows is somewhat in a state of disrepair.