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.
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.