@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