And the fix is in the bug report
Systemd updated on Arch Linux from 246.6 -> 247.1 3 days ago and we have been seeing this 2 min hang on every shutdown/reboot since. Applying @bberg’s single-line fix to add Slice=-.slice in the [Service] section of/usr/lib/systemd/user/gnome-session-restart-dbus.service fixes the issue.
Just add Slice=-.slice to the bottom of this file as root /usr/lib/systemd/user/gnome-session-restart-dbus.service