You can restrict who has the ability to change the visibility of repositories in your organization, such as changing a repository from private to public. For more information about repository visibility, see "About repositories."
You can restrict the ability to change repository visibility to organization owners only, or you can allow anyone with admin access to a repository to change visibility.
Restricting who has the ability to change the visibility of repositories in your organization helps prevent sensitive information from being exposed. For more information, see "Best practices for preventing data leaks in your organization."
Warning: If enabled, this setting allows people with admin access to choose any visibility for an existing repository, even if you do not allow that type of repository to be created. For more information about restricting the visibility of repositories during creation, see "Restricting repository creation in your organization."
In the top right corner of GitHub.com, click your profile photo, then click Your organizations.
Next to the organization, click Settings.
In the "Access" section of the sidebar, click Member privileges.
Under "Repository visibility change", deselect Allow members to change repository visibilities for this organization.