Good news if you plan to dive into Diablo 4 with the Steam release coming on October 17th, as it has been Steam Deck Verified. There's some bad news if you already purchased it on Battle.net though.
For Diablo 4 being Steam Deck Verified, this means everything should just work out of the box directly from Steam and Valve's Diablo 4 verification was done on Proton Experimental. It already worked pretty great on Steam Deck anyway, as I showcased previously.
The bad news? Well, if you were hoping to simply transfer your Battle.net purchase over to Steam, it's a solid nope. Even though Steam keys cost developers / publishers nothing, Blizzard aren't going to provide you with one. As mentioned on X (formerly Twitter) by Adam Fletcher, the Global Community Development Director on Diablo, in reply to a user asking about having to buy it again Fletcher replied: "Yes. Two different store fronts run by two different companies."
Why don't some publishers do this? The reasoning is pretty simple really: Valve take a cut of all sales on Steam, including DLC and micro-transactions. So if you purchased directly before, publishers will want to keep you there so any extras you purchase don't get a cut eaten by Valve.