Comment by toast0

Do you need a guarantee or is enough that it's painful enough for the BSD maintainers when they remove syscalls that they rarely do it? It's even worse if they renumber them so that really doesn't happen outside of syscalls that were only briefly available in a development branch.

Varies a bit by flavor: OpenBSD values security more than stability, so they are willing to break old binaries more often; FreeBSD does require compat modules/etc for some things, but those are available for a long time and sometimes something slips through.

If they break old syscalls, it breaks your code that skips libc, but it also breaks running an old userland with a new kernel and that needs to work for upgrade scenarios. It also breaks binaries that were statically linked with an older libc. When a new kernel breaks old binaries, people stop upgrading the kernel and that's not what maintainers want.