There's an inconsistency that bothers me in the current proposal: apparently the original motivation was to fix the lockups and delays caused by lingering processes, but it also claims to address the security of processes outside of a login session.
ISTM there are two things that might be reasonably configured: 1. Is a given user permitted to create processes that persist beyond logout? 2. If a user may create processes that persist beyond logout, *which* processes persist beyond logout? I would argue that (1) is a reasonable thing for an admin to be able to configure, but I think it should be configured in one place, not two. I would argue that (2) has nothing to do with admin-imposed policy and is simply a matter of what the *user* wants and should be done with reasonable APIs.
For a user who is permitted to create persistent processes, nohup, tmux, screen, etc should absolutely create persistent processes. gconf, ibus, pulseaudio, etc should not. Presumably, things in non-default scopes should persist [1], as should explicit services that aren't configured to automatically shut down. Anyway, here's an actual idea: could systemd and GNOME arrange for terminal programs (things invoked in gnome-terminal, via ssh, etc) to persist and things that are graphical or dbus to not persist? For example, GNOME could stick everything into a scope that is killed when the GNOME session ends, gnome-terminal could split its children into a different scope, and ssh sessions could have a scope that always lingers (if permitted)?