D3DX is a user-space library, though. Separate from D3D. So if WINE or VKD3D ship their own open-source version of D3DX9, you could use that, but you could also

D3DX is a user-space library, though. Separate from D3D. So if WINE or VKD3D ship their own open-source version of D3DX9, you could use that, but you could also use the original Microsoft version of the .dll - the individual numbered versions (d3dx9_31.dll, etc) exist to facilitate using the same version that your game was compiled and tested against. The D3D shader compiler (D3DCompiler) dlls are still versioned in this way for games to link against as well. If you look inside the SxS directory for a current installation of Google Chrome, you'll find a D3DCompiler_47.dll sitting in there that they deploy instead of relying on whatever the OS has available.