Although Microsoft is determined to buy Activision Blizzard, which would give the company full ownership of the Diablo IP, probably resulting in the game eventually becoming available on Game Pass, the acquisition is yet far from being closed and there is a chance that the deal will be blocked by regulators.
So until Microsoft completes all the regulatory requirements, Activision Blizzard will continue to operate as an independent entity. And for now, the company has chosen not to add Diablo IV to the subscription service.
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