At this time, it seems if you are a citizen of Europe, departure from Schengen area, you can travel to Budapest for a transit flight until 24h to the next flight if you stay at the airport. For example I'm french go to Budapest with air France and show my flight to Rio with a stop in Germany (Hungary is not on risk countries of Germany). So there's no problem for now until new changes.. will book for October 2020.
You can use this tool from Air France: airfrance.traveldoc.aero
- transit passengers with a connecting flight, or transit passengers who will leave Hungary within 24 hours by roads prescribed by Hungarian Police, provided they have complied with the entry requirements of their destination countries