The second game from Geert Verhoeff that's only meant to be played on a Steam Deck is Game With Balls, a collection of handcrafted mazes to gyro through. A demo is available for the game right now to get a taste of it.
Check out their teaser below:

Direct Link
More about it:
Game With Balls! A little collection of handcrafted mazes for the Steam Deck, where you use gyro movement to navigate one of the balls through them. Are you the one with the best time among your friends—or even in the world?!
Over 40 levels handcrafted in the real world using real wood, drawings, and other materials, then converted for your digital pleasure.
An additional 10 challenges that not only test you on the time it takes to complete each level but also on other objectives.
Keeps track of your fastest times using Steam Leaderboards, so you can see if you’re the quickest among your friends—or the fastest in the world!
Collect all secrets in every level to not only be the fastest but also the ultimate completist.
Original soundtrack to relax to (or maybe not when you’re trying to beat your friends’ times).
Interesting to see, although probably only a small market for such a thing. Previously they released Better With A Friend, collection of hand drawn mini-games for the Steam Deck which only has 10 user reviews on Steam.

As always: support for the game is the most important thing, not what specific platform the executable was compiled for.