1) Anything that requires MFA via Apple.
2) Full backup/restore will not restore old versions of the apps. New versions are either unavailable or don't work.
3) Any popular streaming service such as Netflix/Amazon Prime/AppleTV.
2. You can manually go in and download “the last compatible version”
3. Netflix is usually good about supporting old devices. Amazon disabled an older auth protocol. Apple TV+ videos can be accessed via the web (tv.apple.com). While Safari seems to always redirect you to the app, Chrome for iOS doesn’t.
2.) This doesn't work when the app has been removed from the app store.
As an aside, I threw away my 1st gen Apple TV over a year ago because Apple bricked it. It needed MFA to sign in (because it timed out?) and only accepted four digits.
The old apps still install just fine and we’ve never had to make any effort to maintain it, or fiddle with operating systems or software or anything.
It’s hard to imagine a better value from a device like this, especially when you take into account the amount of time you would have to spend managing something that had more software freedom to it.