Some of you have probably seen the blog post a few months ago about how GNOME is more strongly depending on systemd. The changes mentioned there have landed into the latest stable versions of the mentioned software (GNOME 49) and do affect us. In particular, the main culprit is the removal of the non-systemd fallback code in gnome-session. This makes it currently impossible to launch gnome-shell/mutter on a non-systemd system. A fairly straightforward patch of using elogind, like what was previously done, no longer works either.
Since we don’t have the time or interest to write a new non-systemd codepath for gnome-session, this means that all support for gnome-based desktops has to be dropped. In particular, the affected packages would be gnome-session, gnome-shell, mutter, and gnome-settings-daemon. For now, the old versions are still in the repos but because there is so much intertwining between other gtk/gnome packages, there is no guarantee they actually work and will later be removed from our repos.
Standalone gnome applications will still continue to be packaged, but it is simply not feasible anymore to support gnome desktops without systemd.
That isn’t quite true. There have been several proprietary implementations for non-Linux systems—Apple’s XQuartz was still being maintained as of a couple of years ago, although I don’t know about its current status. Standards documents exist, and anyone can code to them.