Eu tive sucesso usando
$ busctl get-property org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager ScheduledShutdown
Por exemplo:
$ sudo shutdown -h +30
Shutdown scheduled for Wed 2018-10-10 09:33:44 UTC, use 'shutdown -c' to cancel.
$ busctl get-property org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager ScheduledShutdown
(st) "poweroff" 1539164024057786
$ sudo shutdown -c
$ busctl get-property org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager ScheduledShutdown
(st) "" 0