I don't have any experience being deported, so I can't say for sure, but I just asked my wife, coincidentally, she was a former flight crew (she used to work with SAS and Qantas) and a lawyer. She said you need a visa to board a plane; you can't just get on a plane without a visa or authority to travel, or the check-in desk wouldn't board you. And if you check in without any valid travel document, they would call immigration on you.
If you were deported, you can only get on a ticket to your country of origin, and CBP would have an agent escort you to a flight and make sure you got on. You can't get on a flight to Germany, and this is from befor Trump, they will probably lock his arse up if he show up in an airport without status.