That strategy is described here, and comes from game developers wanting DirectX to be untied from OS versions as it used to be.

https://devblogs.microsoft.com/directx/gettingstarted-dx12ag...

The Agility SDK is unrelated to what is being discussed here. It is a delivery mechanism for the Direct3D 12 runtime instead of DXGI, it is opt-in from the application side, and is done through a more stable loader stage in the core OS instead of hot-patching.

It does, however, give insight into the situation that the DirectX team is in. The in-box version of D3D12Core in the latest version of Windows 11 is SDK version 612. This can be compared against the released Agility SDK versions:

https://devblogs.microsoft.com/directx/gettingstarted-dx12ag...

SDK version 612 is just before the 613 Agility SDK release on 3/11/2024. This means that the version of DirectX 12 they are able to ship in the main OS is a year and a half behind the latest released version.