As announced last October, Valve has been reviewing the entire Steam catalog for compatibility with the Steam Deck, and the first batch of compatible games has appeared online. As noted by a user on the r/SteamDeck subreddit, Valve has started updating store listings with Steam Deck compatibility notices, as can be seen by a search on Steamdb.
Valve has been breaking down Steam Deck compatibility into four categories: Verified, meaning it runs perfectly on the Steam Deck; Playable, meaning it might need some tweaks to run correctly; Unsupported, meaning it won't work at all on Steam Deck; and Unknown, meaning Valve simply hasn't checked it yet. Some notable titles that are verified as of now include Dark Souls II, Dark Souls III, Sekiro: Shadows Die Twice, Death Stranding, and Portal 2. The list is still being updated however, so keep checking this link to see what games are being added.
After a controversial delay close to its original release window of December, Valve recently confirmed that the Steam Deck is still on track to launch in February. When the Steam Deck compatibility program was first mentioned, Valve announced there would be a way for users to check their libraries compatibility before launch. There has still been no updates on that program, but these titles being updated could be the first signs of an official announcement by Valve.