That’s a sad message, @robert.ancell, even if I of course respect the decision to push the guest session into the background when prioritizing. Many thanks for the efforts you made.
But LightDM is still available and used by most (all?) flavors, and the guest session can be used. I just confirmed that it can be used also on an updated standard Ubuntu bionic after having switched to LightDM. So those who want to use the guest session feature can still do so.
Then there is the big but: On post 16.04 versions the guest session is not confined by AppArmor, so the original security model is broken. For some use cases this is something you can live with, while it’s currently considered impossible in other cases to use the guest session feature on higher versions than 16.04.
Balint Reczey has filed a bug report with a request to adapt the AppArmor profile to systemd. Not anybody can fix that bug. I suppose it needs someone with deep knowledge of the system.
So, now when we maybe won’t see a guest session feature in GDM for the foreseeable future, is there possibly a chance that some developer can be assigned the task to fix the AppArmor profile for LightDM?