I think DLCs are becoming a thing of the past in general. Usually the data for the DLC comes with the main game, you just buy a license to unlock it. I can’t remember the last time I bought a DLC and hat to download something additionally or update my game. I’m not a fan of it, but this is where we are going. This just means that wherever you bought the main game from, you will also have to buy the DLCs, since companies will never accept to share licenses between each other. This is not a Steam issue, this is a developer issue.