The Ubuntu team is following Fedora's example and dropping GNOME's X11 session [distrowatch.com] in the distribution's next version. The announcement [ubuntu.com] for the change reads, in part:
"The login screen (powered by GDM) will no longer offer the Ubuntu on Xorg option. All sessions based on GNOME Shell and Mutter are now Wayland-only and users who rely on X11-specific behaviors will not be able to use the GNOME desktop environment on Xorg. We understand that some users still depend on Xorg's implementation of X11; for example, in remote desktop setups, or highly specialized workflows. If you require Xorg specifically, you can install and use a non-GNOME desktop environment. Xorg itself is not going away, only GNOME's support for Xorg."
- https://distrowatch.com/dwres.php?resource=showheadline&story=20007 [distrowatch.com]