Anyone with admin permissions to a repository can enable or disable the automatic deletion of branches. Branch protection rules and repository rules can also prevent branches being automatically deleted. For more information, see "About protected branches."
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On GitHub, navigate to the main page of the repository.
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Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
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On the "General" settings page, you can find a section called "Pull Requests". Under "Pull Requests", select or deselect Automatically delete head branches.