How to keep signed commits verified if an employee deleted a GPG key on Github?

Question

GitHub

Hello reader!
The organization uses GPG keys to sign commits. Each employee has his own signature, and the signature verification key is added to the user's github account. If an employee removes his GPG key from the account, all commits that he made earlier will become unverified.
Question - how can an organization guarantee that none of the employees will delete a key from their GPG account? Or what needs to be done so that he can delete it, but the commits made and recognized earlier do not lose the verified status?