The delete_user action/hook can be used to perform additional actions when a user is deleted. For example, you can delete rows from custom tables created by a plugin.
This hook runs before a user is deleted. The hook deleted_user (notice the “ed”) runs after a user is deleted. Choose the appropriate hook for your needs. If you need access to user meta or fields from the user table, use delete_user.
Users deleted from Network Site installs may not trigger this hook. Be sure to use the wpmu_delete_user hook for those cases. The deleted_user hook is called in either case.
Example migrated from Codex:
In this example we send a friendly message to a user before their account is deleted a few milliseconds later.
Example usage
Send email notification when a user is deleted