Not sure about other BSDs, but FreeBSD does have both stable syscall interface, and stable libc interfaces, which keep ABI both across major, and minor versions

Not sure about other BSDs, but FreeBSD does have both stable syscall interface, and stable libc interfaces, which keep ABI both across major, and minor versions. Situation for non-C languages is pretty similar to what you'd expect in Linux, there's nothing second class there.