Usually, if someone deletes a repository, it will be available on disk for 90 days and can be restored via the site admin dashboard. After 90 days the repository is purged and deleted forever. If you place a legal hold on a user or organization, repositories they own are available for restore indefinitely.
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Sign in to your GitHub Enterprise Server instance at http(s)://HOSTNAME/login.
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From an administrative account on GitHub Enterprise Server, in the upper-right corner of any page, click .  
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If you're not already on the "Site admin" page, in the upper-left corner, click Site admin.  
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In the search field, type the name of the user or organization and click Search.  
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In the search results, click the name of the user or organization.  
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In the upper-right corner of the page, click Admin.  
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In the left sidebar, click Admin.  
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Click Place legal hold. 