Why would you think that nobody has ever used Rust there ? Somebody has obviously put in the work to support that platform, rustc doesn't just blindly inherits

Why would you think that nobody has ever used Rust there ? Somebody has obviously put in the work to support that platform, rustc doesn't just blindly inherits the list of target triples from llvm. While it's safe to assume that C gets a decent amount of use on every platform, you can't expect all platforms to be as well-supported as the major ones. Undoubtedly, some of those platforms would be listed as low-tier if the C compilers cared to maintain a platform tier list. But a platform being low-tier doesn't mean you shouldn't use that compiler there. As for trusting Rust or C on niche platforms, C is so full of UB, platform-specific choices, and vendor extensions, that it's hard to ever fully know how well this or that project will work. Rust is much less surprising, if it works at all I expect it to fully work. I'd definitely pick Rust on niche platforms if I have the choice.