"follow" leaks repository information when creating private repo
Using the web interface to fork an existing repository to a private copy, other users receive notices containing the name of the new repository.
In my case, I forked from the team lindenlab/viewer-release to a repo in another hidden team account. Users not in either team received notices containing the name of the hidden team and the newly created hidden repository. Since they could not see either the team in which the repository was created, nor the resulting repo, they should not have received any notice.