The windows driver has 2 paths, the internal compiler, and the same LLVM as in the open source amdvlk release (though there might be things like not-yet-upstreamed changes, experimental new hardware support etc. that differ from the public version, it was fundamentally the same codebase). The same for DX12 (and any other driver that might use their PAL layer). If you want to confirm you can see all the llvm symbols in the driver's amdvlk{32,64}.dll and amdxc{32,64}.dll files. From what I remember, the internal compiler path is just stripped out for the open source amdvlk releases.

I believe the intent was to slowly deprecate the internal closed compiler, and leave it more as a fallback for older hardware, with most new development happening on LLVM. Though my info is a few months out of date now, I'd be surprised if the trajectory changed that quickly.

AFAIK the closed source shader compiler was/is also available for Linux in the amdgpu-pro package, just not in the open source releases.