IIRC Clang on Windows also announced a similar thing (supporting only Win 7+) maybe they plan on building it with Clang on all platforms and it's a result of that.
I think that Clang generated binaries won't run on Windows versions below 7, if I remember correctly it has something to do with implementing SEH but I could be completely wrong and it could also have nothing to do with their decision but it makes sense - they moved to clang for compiling Linux binaries to simplify build process and they are a big driver behind Clang on Windows.
I'd be very surprised to hear this. Clang and Rust both use LLVM, and Rust can generate binaries for XP (with a few caveats). Of course, the fact that LLVM no longer supports running on XP means that the Rust compiler itself doesn't run on XP (not that it ever did).