I personally used Lutris:

Grab the battle.net installer from the lutris website, optionally extend it with the D4 installer (it just creates a second shortcut using the same battle.net install you just did).

Once bnet is installed, you can add Lutris games as shortcuts in steam (you may need to do it from desktop mode without steam running) by right clicking on games and clicking add to steam.

From there you just need to install D4 in the bnet client and away you go.

I followed this guide and it works great! https://www.gamespot.com/articles/how-to-play-diablo-4-on-steam-deck/1100-6514752/

I am gonna buy it and I've used Bottles to set up the Blizzard game launcher. I already use it to play Overwatch so I'll just install Diablo the same way.

I have been playing for a couple weeks on SD with no issue. I forget which guide I used but the steps were basically:

• add battle.net installer as a non steam game and toggle proton
• install battlenet and diablo
• add diablo.exe as a non steam game and toggle proton

And you are done. Some times you have to double click the battlenet window to go Fullscreen so you can see the play button