After you merge or close a pull request, you can delete the branch. This keeps the list of branches for your repository as clean and useful as possible.
Tip: You can only delete branches for repositories that you have push access to.
You can also delete branches from the Branches page. For more information see Viewing branches in your repository.
Restoring a deleted branch
If you accidentally delete a branch that you still need, you can restore any head branch of a closed pull request.