The cause of the crash is out-of-range access to the array.
ExcludeDecals eliminates the mesh instance to which the decal material is assigned, but it seems that the MeshCommand has been registered.
The relevant part is in the GatherRayTracingWorldInstances function of DeferredShadingRenderer.cpp.
There is a part of this function that checks the ExcludeDecals flag, but I have confirmed that the crash can be avoided by moving this branch instruction before the previous command addition process.
I have attached this settings project.
Setting r.RayTracing.ExcludeDecals 1 in this project will reproduce the crash.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-99710 in the post.