It's not that hard. They just remove all of your agency as a user. You can push commits, open branches & pull requests, and merge if 2 people approve it. And that's it.
Want to merge? Restricted. Make a new repo? Restricted. Use a GitHub Action? Restricted. CODEOWNERS? Restricted. Branch filters? Restricted. Forks? Restricted. Releases, packages, artifacts, security, insights, settings, webhooks, environments, pages, wiki, issues? Restricted. Access a repo you aren't a member of? Restricted. Protected tags, dependency graph, dependabot, code scanning, secret scanning, deploy keys, secrets, github apps, oauth, notifications? Restricted. Stars? Restricted. And your SSO token expires every hour.
Can't get hacked if you can't do any work!