A year and a half ago, shortly after the GNOME 45 release, I opened a pair of Pull Requests to deprecate and remove the X11 Session. A lot has happened since. The GNOME 48 release addressed all the remaining blocking issues, mainly accessibility regressions, but it was too late in the development cycle to drop...
I know, right? Quite ironic that their own devs were going on about how unhappy they were with GDM dropping X11 session support.