The deregistration certificate (Abmeldebescheinigung) confirms that you have deregistered your address in Germany. You get this document when you do your Abmeldung. If you do your Abmeldung by post or by email, the Abmeldebescheinigung is sent to your new address.
If you already left Germany, you can email any Bürgeramt and ask for an Abmeldebescheinigung. They will send it to you by post. This costs 10€.