WoW disabling Aero on Windows 7 when running on Direct X 12 is an intended change.
There are several edge cases where DX12 on Win7 can get into a bad state, which are limitations of Windows 7 that Microsoft doesn’t handle in their DX12 layer (which they do handle in their DX11 layer). These edge cases include things like unplugging the monitor or launching the game from a GPU that doesn’t have a monitor plugged into it. These are only issues if you’re using the Aero theme. The basic theme doesn’t have these edge cases.
In 8.1.5, we only disabled the Aero theme if we detected cases like running on a GPU that doesn’t have a monitor plugged into it. We were getting crashes from 8.1.5 still and realized other cases, including things like unplugging the monitor were much harder to handle after the fact. To prevent crashes, we changed to always disabling Aero theme on Win 7 DX12. The motivating case for the change was if someone was playing on a laptop with an external monitor, unplugging the monitor would crash the game. It wouldn’t be hard to add a config file setting to disable disabling the Aero theme on launch, but you’d be opting into crashing in some edge cases.