I ran into this problem a little while back trying to get my Linux implementation of the BSD unveil() system call merged into mainline. Some of the responses to the RFC told me that it shouldn't be added because glibc likely won't add a syscall wrapper for it. However, a response from glibc states that they won't consider adding it until it's been successfully merged into mainline, creating a sort of catch-22 situation.
This is an idiotic stance from maintainers. Linux already has a lot of things that most currently used libc implementations don't support, like getrandom.
