There's currently somewhere around 68,000 games on Steam and so it's going to take a long time for Valve to check them all on Steam Deck but here's some recent picks.
There's a bunch that doesn't work for my Steam Deck.
It does open. It does play. But controllers dont map correctly or there's weird layering UI issues, where the game is unresponsive because its waiting for a keyboard event somewhere else, and the player can't actually get there using a controller because the devs assumed people would only use mouse+keyboard. Not even switching controller setups make it work.
That's my bad; I automatically read "Steam Deck" in the parent comment as "Linux" which is obviously a much different story. I've definitely had my share of issues getting certain games to work properly on my Steam Deck that otherwise run flawlessly on my desktop.
Only 13% of my Steam library is verified. That's still plenty of games, but it's a lot more limited than "all games on Steam." More than half of the top 20 games on Twitch are unplayable or run terribly on Linux.
It opens some doors if you're willing to accept "playable" games. That's another 14% of my library. The vast majority are a crapshoot for me on the Deck. Most of the issues revolve around text illegibility and clunky controls.