UPDATE: Using GE-Proton7-27 gives an error for an unsupported graphics card, but allows you into the game by pressing OK.
You can see our gameplay footage here
At the time of writing, the Diablo 4 beta is not working on the Steam Deck with Steam OS.
There seems to be an issue with the detection of the Steam Decks APU which is blocking the game from launching.
We are awaiting testing on Windows 10 on Steam Deck but due to the authentication server issues and queues, this might take some time. We will update this article with further news.