The files are located at /srv/salt/omv/deploy/ssh/, but you should not touch that files, otherwise you work against the package management and how OMV works. Simply add another Salt state which includes the changes you want to apply to be on the safe side.
This was a good hint that helped me achive the same banner goal, yet it feels more of a hack. I think it would be a good idea to include the SSH banner setting in the GUI aka Workbench, at least for the upcoming 7.x.
pi@omvhost:~ $ cat /srv/salt/omv/deploy/ssh/30sshd_banner.sls
/etc/ssh/sshd_config:
file.append:
- text:
- Banner /etc/issue.net
pi@omvhost:~ $
I ran omv-salt deploy run ssh && systemctl reload sshd and I was done with it.