An update for Rise of the Tomb Raider was released yesterday, adding support for the DX12 API: http://steamcommunity.com/games/391220/announcements/detail/870690772757808963
As you can see:
Adds NVIDIA VXAO Ambient Occlusion technology. This is the world’s most advanced real-time AO solution, specifically developed for NVIDIA Maxwell hardware. (Steam Only)
So it seems like this is the first game which uses some actual DX12_1 features. VXAO uses the new conservative rasterization feature of DX12_1, which I personally mentioned as one of the most important new features in DX12 on various occasions, since it allows new/more advanced rendering algorithms. It will be interesting to see how this feature is received by the general public. It will also be interesting to see how many other games will follow this example.
Edit: According to this article on Tech Report, the VXAO feature is only available in DX11 mode, which would imply that it uses DX11.3, which gives access to the same DX12_1 features, through the older API.
This post from Nixxes on the Steam Community forum seems to imply that support for VXAO in DX12 will be added later:
You should see VXAO as the rightmost ambient occlusion option, assuming you are on Maxwell hardware, have up to date drivers, and are running on DX11. VXAO is not yet available on DX12 from NVIDIA.