LEGO Voyagers is the cutest looking co-op game and it's available today, along with it getting Steam Deck Verified for the release. So it will work great on other Linux / SteamOS systems too. Created by Light Brick Studio, who also made LEGO Builder's Journey.
The initial release had a bit of a blip with people not being able to play, as it was showing as the Friend's Pass version, the developers have already solved this before publishing the article. The Friend's Pass is a nice system, allowing you to jump into the game with a friend, so only one of you need to purchase it. I wished my co-op games had something like it.

Direct Link
More about it below:
When two friends make it their mission to rescue an abandoned spaceship, they embark on a journey beyond their wildest dreams, ultimately learning the value of being connected.
Experience what it feels like to be a LEGO brick. Tumble, jump, snap together, and build your way through rich brick worlds.
Unravel a poetic, non verbal narrative, set in beautiful brick built environments, supported by an atmospheric soundtrack.
Cooperatively solve the game's many challenges and discover how two bricks are better than one. Play locally on the same screen, or online.
- Invite a friend to join your adventure for free with the Friend’s Pass. As long as one of you owns LEGO Voyagers, you can both play the full co-op experience together.
Really need to get finished with our play-through of It Takes Two, as this looks wonderful and will no doubt be the next game we co-op over here.