I have no philosophical complaints with supporting odd architectures in general. I agree that most obscure targets are probably not that much code, since the library is factored with this in mind (e.g. basic WASM support took an afternoon).

It's stated as a non-goal simply because it's not the most valuable thing I can do with my time. My fundamental stance is that writing new Windows or Linux or macOS or WASM programs in C is a good idea, and those are the programs that I write, so that's where my focus is. But if someone would like to come along and write the ~30 syscalls needed to port the library to a new platform, or even register any interest in such, I'd be happy to look into it at that point.

Aren't these MCUs predominantly ARM-based, with some RISC-V thrown in?

I see no contradiction in the desire to support x64 (because it would be ridiculous not to), ARM, and likely RISC-V, but not the venerable but now-fringe architectures like MIPS or Sparc or 68040 or even x86.