If user list is disabled ("disable-user-list=true" in
/etc/gdm3/greeter.dconf-defaults), TimedLogin fails to do anything.

It seems that the functionality is bound to the user listing, which is
unfortunate because I consider the said functionality to be a security
vulnerability...

It seems to me that TimedLogin should work regardless, or at least that
the behavior should be documented. I looked here in the documentation:

https://help.gnome.org/admin/gdm/3.14/configuration.html.fr

Thanks!