Some sessions aren't shown any more
This is the session layout:
laney@disco:~$ tree /usr/share/{wayland-,x}sessions/
/usr/share/wayland-sessions/
├── gnome.desktop
└── ubuntu-wayland.desktop
/usr/share/xsessions/
├── gnome.desktop -> gnome-xorg.desktop
├── gnome-xorg.desktop
└── ubuntu.desktop
And without any evidence at all I'm blaming c98ea6a4 (although that's weird since it de-dupes based on translated name, and those are different - the one that's missing is called "GNOME on Xorg").
Looking into it.