My machine has elogind and slim running (devuan default behavior), and from what I can tell this gives you the same functionality as xdm used to, but worse.

I'm not sure why things got split into two subsystems, but I guess logind is essentially required in systemd systems. I remember there were a bunch of bugs where systemd would do stuff like unexpectedly force-kill background processes, and rm -rf home directories, etc, etc.

I think those bugs had something to do with logind, but I'm not sure.

Again, I just see weird vestigial stubs on my machine, not the underlying multi-decade train wreck.