Ah yeah, and it looks like this is what they're using: https://github.com/microsoft/D3D9On12
And it looks like DirectX 11 to DirectX 12 translation exists as well: https://github.com/microsoft/D3D11On12
Microsoft seems to have provided some of that in the OS as open source here:
https://github.com/microsoft/D3D9On12
Unfortunately, it looks like it's up to the application developer to support it, unless I'm reading it wrong.
> rewrite d3d9.dll to emulate the API in user mode on top of D3D12.
There seem to be such a project actually, just released to open source:
https://github.com/microsoft/D3D9On12
https://devblogs.microsoft.com/directx/open-sourcing-direct3...
But it seems you have to actively enable it in the development time, the OS won't just silently do this on itself and I doubt Microsoft or anybody else is going to update their software to do so.