"drop support for X11" isn't really true

I think describing this as dropping support for X11 is misleading. The linked Ubuntu forum post says "drops support for GNOME on Xorg", and the linked GNOME blog post is about dropping the "X11 GNOME Session" - both of those are more accurate ways to describe what is being dropped.

X11 is a protocol, not an implementation, and I expect that it will be supported for a long time. In a Wayland session, the Wayland compositor normally runs Xwayland, an X11 server (either on startup, or on-demand). X11 clients connect to Xwayland, which acts as a Wayland client on their behalf (with a varying level of functionality when compared with the Xorg session, depending what exact features the X11 client wants to use), while native Wayland clients bypass Xwayland and talk directly to the Wayland compositor.

Xorg is an implementation of the X11 protocol as a top-level display server that talks directly to the hardware. The thing that Ubuntu and upstream GNOME are both interested in dropping is the desktop session labelled "GNOME on Xorg", which uses Xorg as the X11 server, and runs the GNOME window manager, compositor and applications on top of that.

The possible regressions here are:

- if the combination of Xorg and GNOME's X11 compositor works as intended on a particular hardware/driver combination, but GNOME's Wayland compositor does not;
- or if an X11 application is relying on a particular X11 feature that Xwayland cannot emulate faithfully, most commonly because Wayland security restrictions don't allow it or because Wayland doesn't have a protocol for it