The issue is with the issues, which are not cloned together with the repository (and pull requests, while there's the trick of using --mirror to also clone the pull requests, the comments aren't cloned).
There probably is, and if not, someone will have created one. But even then, "every developer has almost or exactly an identical copy of the repository in at least one location" is false for issues and pull requests; while nearly every developer can be assumed to have one or more "git clone" copies of the repository, few will have any copy of the issues and pull request comments.
I don't think GDPR comes into this. You could probably make the case that the issues you've posted yourself are personal/identifying information but github have no obligation as far as I can see to allow you to export issues and comments created by others as well.
