GDM has supported sessions registering with it for a few years now so it can know when to shut down the greeter. Having the GNOME Classic session declare that it will register itself allows GDM to avoid executing a fallback codepath. This has been supported with the regular GNOME session for a while, and this session was likely forgotten about when it was added there. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/195>