“Linux + SteamOS” compatibility is a bit legacy. It mostly refers to games that have a native linux client (I believe some of the early Proton Validated games are included as well.
The Steamdeck (and generally speaking, any linux system) uses Proton as a compatibility layer with windows, and the “Steamdeck Verified” system is more relevant today. That said, even the Steamdeck verified system isn’t perfect. There are a number of titles that, while verified, have some problems with the deck, typically later in the game or after running for some number of hours. There’s also a vast number of games that while not “Steamdeck Verified” work perfectly on the deck and linux via proton (though you do have to enable it in the settings).