Suddenly unable to access openHAB

It means you are not running the release version of openHABian. You are running some other branch. If you want to keep up with development of openHABian, I recommend sticking with “release”.

You are configured to run the latest release of OH. OH 4.0 was release this weekend. If you did not want to upgrade when OH releases new versions you would need to fix the version number. I don’t know if that can be done in openHABian which is always going to give you the latest version of whatever branch of OH you are running: release, testing, snapshot.

sudo apt-mark hold openhab-3.4.2 
sudo apt upgrade

I think that’s how it’s listed. You should do an apt-cache search openhab if that doesn’t work.

That does seem to be the case for most.

It’s definitely right. It’s just 3.4.2 isn’t the latest release any more. It’s been replaced (three times over, the latest OH 3 is 3.4.4, and as I said OH 4.0.0 was release this weekend).