I guess you’ve seen this already:

$ ssh root@nas.local
Activate the web console with: systemctl enable --now cockpit.socket

Last login: Sat Jan 11 13:05:33 2020 from 192.168.1.137

# _

That’s why you are here, is it? Well, easy help:

rm -f /etc/issue.d/cockpit.issue /etc/motd.d/cockpit

Problem solved. If you want it back, simply reinstall cockpit package. I am not sure if upgrade would put them back or not tho.