This was somewhat done for Destiny 2, when they moved from Battle.net to Steam (so like the Bethesda situation mentioned in another reply). I’m sure it was a bit easier for them since the base game went F2P which obviously hasn’t happened here for Diablo 4, but all of my purchases/DLCs for the game were transferred over to the Steam equivalents.

I’d assume if they really wanted to, they could find a way to just give everyone who has it on BNet a key to the Steam version, but there’s no real incentive for them to do that.