Other projects also make use of alternate namespaces. The oft-forgotten built-in "git notes" puts stuff in the `refs/notes/` namespace (specifically in `refs/notes/commits`). Gerrit uses the virtual `refs/for/` namespace for receiving commits for review, stores project config in `refs/meta/config`, and stores User data in `refs/users/` in a special repo [2]. I'm sure others do interesting things.
Alternate uses of git's DAG model are fascinating.
[1] https://github.com/git-bug/git-bug/blob/bd936650ccf44ca33cf9...
[2] https://gerrit-review.googlesource.com/Documentation/config-...