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.
Because I bought the game? (Well I suppose it’s really renting it now)
Why should it matter which launcher I use? It says I need a battle.net account so I don’t see why it wouldn’t just work since they know I already bought it.
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.
Because I bought the game? (Well I suppose it’s really renting it now)
Why should it matter which launcher I use? It says I need a battle.net account so I don’t see why it wouldn’t just work since they know I already bought it.
This is answered in the OP article itself: