I definitely see the need of splitting, but I find this one harder (or simply, much more effort) to split in any meaningful way as I'm not very familiar with their actual use.
I'd suggest something like this:
- D3D12_COMMONSHADER_*; Common shader constants. - D3D12_CS_*; Compute shader constants. - D3D12_DS_*; Domain shader constants. - D3D12_GS_*; Geometry shader constants. - D3D12_HS_*; Hull shader constants. - D3D12_IA_*; Input Assembler constants. - D3D12_PS_*; Pixel shader constants. - D3D12_RAYTRACING_*; Ray-tracing constants. - D3D12_REQ_*; Minimum resource limit constants. - D3D12_VS_*; Vertex shader constants. - Miscellaneous constants.
That probably leaves "Miscellaneous constants" a bit larger than it strictly needs to be, but small enough that I'm not too bothered about it.