@BrodieOnLinux Hello. You didn't go into much detail, and I see your audience commenting about it, so:
I think it's pretty thoroughly decided that GDM will retain the ability to launch non-GNOME X11 desktops. Luckily this functionality is isolated from the rest of GDM, as noted in the issue
What will be going away is: "legacy X11" (where one X server running as root is shared by the whole system), XDMCP (ancient remote login for "X terminals"), and GDM's ability to run its own UI under X11
@AdrianVovk It's good to see that's the direction that was chosen, there's still the matter of whether or not Ubuntu is going to keep X11 support in GNOME for the next LTS release with upstream most likely dropping it.