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We made pam_systemd module required, based on [1]. However, this was unfortunately not discussed by systemd maintainers and it turned out there are reasons to keep the module optional.
Quote Michal's comment [2]:
"I don't think that having pam_systemd as "required" is a good idea. RHEL components (certainly dbus-daemon at least) which come into play here are not resilient enough to support this. logind relies on dbus-daemon and on having functioning system bus connection. However, it is not uncommon to see systems where dbus-daemon is updated and restarted while dbus services are not touched (logind included). On other occasions we saw logind getting kicked out of the bus by dbus-daemon due to bugs. Granted, such system is not really in a fully functional state but you can still log in and correct the issue, e.g. reboot or restart all dbus services. When the module is "required" this is not possible, all future login attempts will fail and only way out is power-cycling the machine.
Another problem is that this change was not co-ordinated with systemd upstream. I've asked Lennart and others for the opinion on this and they still think pam_systemd should stay optional."
Summary:
If logind is not able to use dbus-daemon for some reason (it is stopped, being restarted or there is some bug), users are not able to login and the system is effectively locked out.
Proposed solution is to require the module to be present but keep it optional so if it fails to register user session, user can still log in.
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1643928
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1643928#c11