GitHub has an archived repositories feature to indicate and make unmaintained repositories read-only. You can pair this feature with an automated report that gives you a list of the inactive repositories that are in your organization. Check out that action on the GitHub Marketplace.
In order to archive a github organization repository:
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Edit the README to add a note:
**NOTE**: _This repository is no longer supported or updated by <COMPANY_NAME>. If you wish to continue to develop this code yourself, we recommend you fork it._</code>
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Then open a pull request with that change. Here's the text we've been using for the pull request:
This repository is no longer being maintained. This pull request makes it explicit in the `README.md`. Let me know if you have any reservations, otherwise I'm going to archive this repository. /cc any relevant teams or recent <COMPANY_NAME> committers
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Let it sit until you receive confirmation from a maintainer or an adequate length of time has passed.
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Close all open issues and pull requests and archive the repository from the repository settings.