You don’t need SteamOS, but you do need to enable Proton for all games.
Open Steam and click Steam on the menu bar (upper left corner) and click Settings.
1. Click Steam Play.
2. Check Enable Steam Play for supported titles.
3. Check Enable Steam Play for all other titles and select the Proton version you installed.
4. Press OK.
You don’t need SteamOS, but you do need to enable Proton for all games.
Open Steam and click Steam on the menu bar (upper left corner) and click Settings.
protondb.com has a list of supported titles