OMV 5 should really be /usr merged (not necessarily have usrmerge installed). There are only a few packages on OMV 5 (nut is one) that need usrmerge. And Volker has said he does not want to automatically install usrmerge on upgrades.
in OMV 6 (fresh install) /usr/bin/mount exists (check it out for yourself).
OMV5 don't have it and I don't have never installed usrmerge.
Now /usr/bin/mount it is missing in my system after migration.
Code
root@OMV6new:~# cat /etc/monit/conf.d/openmediavault-filesystem.conf | grep mount
# Try to auto-mount a filesystem if it is missing. Alert if the filesystem
check program mountpoint_srv_dev-disk-by-uuid-652f2774-bceb-43af-af08-a5d53f3a72af with path "/bin/mountpoint '/srv/dev-disk-by-uuid-652f2774-bceb-43af-af08-a5d53f3a72af'"
then exec "/usr/bin/mount '/srv/dev-disk-by-uuid-652f2774-bceb-43af-af08-a5d53f3a72af'"
root@OMV6new:~# ls -al /usr/bin/mount
-rwsr-xr-x 1 root root 55528 28 lug 20.09 /usr/bin/mount
root@OMV6new:~# dpkg -S /usr/bin/mount
dpkg-query: nessun percorso corrispondente a /usr/bin/mount