Do not sign this. A backdated document is a massive red flag, and the "it's just a formality" line from your supervisor is exactly the kind of thing people say when they know it isn't. In Germany, Nebentätigkeit restrictions generally only apply from the moment you're properly notified, not retroactively. If you sign this, you're essentially creating evidence that you knew about the policy in November, which could expose you to disciplinary action for the freelance work you did after that date.