Backwards compatibility down to D3D_FEATURE_LEVEL_11_0, the exclusive new features are not mandatory to use, the new programming paradigms specific to D3D12 remain though.
There seems to be very little D3D9 code in there, just querying some infos or something at first glance...
It does call D3D12CreateDevice here with MinimumFeatureLevel (passed in EDX/RDX register) set to 0xb000, which is defined as D3D_FEATURE_LEVEL_11_0:
OK, but the point that was made earlier was simply that D3D12 library was backported to Windows 7 with some limitations. Nothing more, nothing less. And using only capabilities of D3D11 compatible cards is the valid use case.
You're talking about implicit dependencies, there can be also dynamic dependencies, which is what Vulkan and D3D12 are in RDR2's case.
https://www.dependencywalker.com/help/html/application_profiling.htm