The fallback was removed, it was only necessary when there was no DXR capable card and also to help developers with the new API, the only way to have everything working for old generation GPUs is for each vendor explicitly implement the feature in the drivers and this is why Microsoft left the fallback code public available. As I didn’t study the code, it is uncertain if the code is only fit for DirectX (most likely) or can be used for Vulkan somehow (very unlikely).