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."
On your GitHub Enterprise Server instance, navigate to the main page of the repository.
Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
On the "General" settings page, you can find a section called "Pull Requests". Under "Pull Requests", select or unselect Automatically delete head branches.