Documenting syscalls does not mean a guarantee of stability. I can't recall any other Unixes except Linux which has committed to maintaining syscalls, which means this interface cannot be relied on.
e.g.
OpenBSD deletes "relic from the past" syscall:
https://cvsweb.openbsd.org/src/sys/sys/syscall.h?rev=1.199&c...
Solaris documenting deleted syscalls (note however the libc interface is guaruaneed):
https://docs.oracle.com/cd/E23824_01/html/E22973/gkzlf.html
MacOS X is so unstable that even go developers had to go through libSystem: