The feature does or did exist, but hidden. The early versions of GitHub Enterprise (called GitHub FI) exposed this and it can be seen in action in some places (e.g.: https://github.com/ghc; look at the "mirrored from" subtitles). I really wish GitHub would expose that feature publicly, though. The post-receive advice they give only works if you have full access to the master repo and misses out on recording the "this is a mirror" piece of metadata on the repository, which is useful for people who might try to open PRs etc