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It is still possible to control the visibility (and permissions) of repositories in one of the existing Github organizations, so it may not make sense to take on the additional responsibilities if this is the main use case for a new, separate organization. Please see: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/managing-repository-settings/setting-repository-visibility and https://docs.github.com/en/organizations/organizing-members-into-teams/about-teams
Note that in some specific cases, a separate organization might make sense, such as:
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