So I've forked someone else's repository, made a few changes, submitted a pull request, and my changes made it into the product. Great!
But…what should I do with my forked repository? Is there a compelling reason for me to keep my repository around, or should I go ahead and delete it? I don't plan on making any additional contributions, but if I change my mind I assume I can always just re-fork it.
I'm not really concerned about keeping a backup. I'm more worried about breaking links, losing commit messages, etc.
Best Answer
Old answer:
Deleting forked repositories will erase history from your Pull Requests.
Deleting a forked repository will delete any information associated with your repository. This can retroactively affect any references to your repository, including pull requests that have already been merged. (See Pull request displays "unknown repo" after deletion of fork)
Your comments and commits should be preserved on any pull requests that were associated with your repository, but you will do so at your own risk.
However, deleting old branches after a merge is perfectly safe.
While deleting repositories should be avoided, deleting unused branches is perfectly acceptable. In fact, GitHub encourages you to delete old branches.
Alternatively, if you really don't want to keep them around, you can archive a repository to indicate it is no longer actively maintained.
See also