Related thread on the implementation method for shadow rays using OPTIX_RAY_FLAG_TERMINATE_ON_FIRST_HIT here:
https://forums.developer.nvidia.com/t/anyhit-program-as-shadow-ray-with-optix-7-2/181312/2
Explanations on what not to do with that flag here:
https://forums.developer.nvidia.com/t/optix-7-2-ray-triangle-intersection-failure/178611/4