@sjb3d I've been meaning to figure this out for myself for a bit, since Avoyd's GPU new path tracer uses Vulkan.
Currently I don't rely or use REBAR much, but I'm planning to rewrite my GPU allocation to check for support. I think the best way to do this is to check for a large device local heap accessible to the CPU.
Perhaps worth noting that I have an NVIDIA 4060 on a machine with no REBAR support (no bios support), so such systems do exist.