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.
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.
This is answered in the OP article itself: