Many games classed as unsupported by Valve are only marked as such because they haven't got round to officially checking them.
Yup, especially for older games, devs aren't going to bother getting them verified unless they're still actively supported. As already suggested, check ProtonDB for compatibility.
Chances are, if it runs on Linux, has decent controller support, and is more than a couple of years old (so has relatively low hardware requirements) there's a good chance it will work fine.
