It’s annoying that GitHub doesn’t seem to provide a way to organize repositories — none that I know of, anyway. GitHub Projects seem not to be usable for this. Prove me wrong, anyone?
Anyway, what I’ve done is to address the problem of too many repositories by creating another repository: https://github.com/holmesrichards/Directory . Its only content is a README.md that provides a categorized directory of my repositories. And on my overview page it’s the one pinned repository. Obviously this has to be maintained manually (and will it be? HA!) but it’s something.