I personally would have been surprised to learn that such a thing existed! As a C# and rust developer who has hacked on llvm before, I have to admit that these two languages (and their underlying techs/stacks) are absolutely worlds apart and there is so little overlap between their communities.

I’d have been less surprised if there were a llvm backend for C#, but that wouldn’t exist without an IL LLVM target (because you’d be limited to the language without any (standard) library support.

The first-pass Roslyn compiler is really naive when it comes to optimizations; I constantly marvel at how little optimizations are performed in its first stages compared to what llvm does (the jit is amazingly well-tuned, however). An LLVM backend for C# would make for very interesting learning and research opportunities!