The "implicitly tagged as unsafe" thing is not an ABI issue. Extern declarations are necessarily unsafe because the compiler has to trust them to be correct. As of Rust 1.82, extern "C" functions can be declared safe to call: https://blog.rust-lang.org/2024/10/17/Rust-1.82.0/#safe-item...
As for the "shared-library-centric OS" idea, I'm not sure anyone is really strongly clamoring for this (mostly because of the difficulties in getting adoption of any kind of novel OS at all), so it basically makes sense for the Rust project not to prioritize it. Niko's broader definition of "foundational software" seems more presently applicable. But if such a thing were to be attempted then figuring out a good ABI would be an important step.
As for the "shared-library-centric OS" idea, I'm not sure anyone is really strongly clamoring for this (mostly because of the difficulties in getting adoption of any kind of novel OS at all), so it basically makes sense for the Rust project not to prioritize it. Niko's broader definition of "foundational software" seems more presently applicable. But if such a thing were to be attempted then figuring out a good ABI would be an important step.