What happens if the subscription to kopano one runs out?
Its clear that there won’t be any updates - but will it stay usable at the current state?
You can easily simulate what happens when you remove your license key and restart the kustomerd process (a restart may not be necessary as the service was afaik design to detect changes license files directly).
And the hard version
deactivate the autostart customer service
(systemctl disable kopano-kustomerd.service)
reboot the server and open the web app
No more work possible;)