> [...] Linus has not bothered to write them down himself [...]

He's a kernel hacker and a technical leader. He doesn't write specs for the userspace, that's the least of his concerns. Linux has very strong guarantees on syscall backwards compat - Go doesn't even use libc. This is all by design.

Even the name "GNU/Linux" was something FSF tried to impose on distributions. The distributions being distributions, were free to brand themselves as they willed, which is 100% fair under the license terms.

This ecosystem has always been a bazaar, if that's not for you - use a BSD. (e.g. macOS.)