By default, Steam installs the preferred version of Proton for any given game, especially old ones (it has a per-game install script). If you went to Steam > Settings > Compatibility and overloaded the default Proton version for unsupported titles with e.g. newest GE, it will use that instead. Meaning, you'll sometimes have to force an older version of Proton in game properties. Don't tinker with it, and most old games will continue to run out of the box for years to come.