When a user is removed from the organization, they will lose access to private forks of your organization’s private repositories, but may still have local copies. However, they cannot sync local copies with your organization’s repositories. See https://help.github.com/en/articles/removing-a-member-from-your-organization
Their private forks can be restored if the user is reinstated as an organization member within three months of being removed from the organization. See https://help.github.com/en/articles/reinstating-a-former-member-of-your-organization
- If it's a private repository, all of its private forks will be deleted as well.