I let my flat in Germany. Do I still have to pay tax on it there?

Yes, in principle. Anyone with neither a residence nor a habitual abode in Germany is subject to limited income tax liability under section 1(4) of the Income Tax Act if they have German-source income within the meaning of section 49. That includes, under section 49(1) no. 6, income from letting and leasing where the immovable property is situated in Germany. The property therefore stays within the German tax net even once you no longer live there. Incidentally, long-term letting to third parties is also the circumstance capable of ruling out a continuing residence under section 8 of the Fiscal Code. General information, not tax advice.