According to the very angry and also very stereotypically German Police officer pulling over me and my friend back in 2017, flashing your lights is in-fact illegal. We were doing approximately 140km/h having just passed the police car in the left lane when someone pulled out ahead of us going much slower forcing us to slam the brakes to avoid hitting them. No blinkers were used on their part and we flashed our lights as we had seen and experienced countless times on our trip to let them know we were coming. We had driven all the way down from the top of Denmark, through Germany, France and Switzerland, down through parts of Italy and were then on the way back up through Germany when this happened so we had seen people flashing their high beams like that countless of times, and it seemed like a good practice if you were going much faster than the people ahead.
My friend was driving at the time and luckily got no ticket and kept his license after getting scolded by the police officer for a good 15 minutes.
No idea if it's actually illegal, but considering of angry the police officer was I'm assuming so. We were confused as to why they stopped us at first, and he kept yelling/asking "what is that? Why do you do that?"
Other then that my experiences driving on the Autobahn (and that whole stretch of road) have been pretty good! Very straight roads of decent quality where they seem to use concrete or something instead of asphalt like they do here in Norway. Someone else mentioned the speed limits kept going up and down over short distances, which I remember as well.
I think the most perplexing thing for us driving on the Autobahn is that you can purchase alcohol, even the stronger stuff, at the gas stations along the way, coming from having a strongly regulated alcohol monopoly system here in Norway where anything stronger than 4,7% alcohol is only available at Vinmonopolet, which is only open specific hours and dribks with a lower percentage are also only available in normal stores within certain hours.
This is turning out to be a much longer comment than I intended.