I would try to do some restores of random files. Kind of a "canary in the coal mine" test. If you have problems with restoring some files or folders, then you'll have a problem with doing larger restores.

I agree - I am running my own Samba server and I don't think I'm affected, but it isn't really clear to me how to double check or why Apple's new default broke things in the first place

You could create a timestamp file on your Mac, where your Mac adds a timestamp per day or hour and have a script check that the timestamps appear in your backup.

Hi! OP here. In my case Time Machine stopped doing backups to the server, period, and would keep silently refusing to do so. I encourage you to check backups are happening.

Thanks - I can see why you skipped writing this, backups not happening at all is obviously a failure :). At least if you ever look there ...