From what I understand you have to re-purchase if you already own it on Battle.net, which is pretty frustrating. As playing on the Steam Deck would be much easier in Steam.
I have done this, but it is clunky having to first launch the Battle.net launcher, then launch Diablo IV on the steam deck. It would be much better to skip the launcher. I would hope when you bought the game through steam you wouldn’t be forced to go through the launcher.
By that same argument, I should expect Blizz games on the Xbox platform (or at least game pass). But ActiBlizz has said that while they plan to do that, not to expect it until at least 2024 when the acquisition closes (if it does). Which also raises the fact that ...
MSFT hasn't acquired Blizz yet, so the push to the Steam platform is likely not driven by that narrative yet.