Airline profiles on Facebook. All information supplied by airline representatives, my questions were asked either in private messages, or in the airline public profile.
Every time I asked a general question, in which I never mentioned any particular flight, type of aircraft, or reservation.

My question was: Do you allow to use bluetooth headphones on board (with the phone in the plane mode, of course)?

Austrian Airlines: "Dear Andrzej, unfortunately Bluetooth headphones are not allowed on board. Kind regards, Ivana"

Lufthansa: "Dear Andrzej, yes, you may use your Bluetooth headphones on board. Have a wonderful flight! /Leny"

Singapore AIrlines: "Hi ‎Andrzej, for our A330/B772/B773 aircraft, Bluetooth devices can be used during the cruise phase of the flight only, while for our A350/A380/B773ER aircraft, Bluetooth devices can be used for all phases of flight. If in doubt, do consult our cabin crew onboard your flight, thank you."

Swiss: "Hello Andrzej, that is only allowed on Boeing 777-300ER at the planned cruising altitude as mentioned on the following link https://www.swiss.com/ch/en/customer...afety-on-board You may inquire with our cabin crew should you require assistance on board. Kind regards, Evelyne"

LOT Polish: My question: Jakie są zasady korzystania ze słuchawek Bluetooth na pokładzie Waszych samolotów? (What are the rules for using bluetooth headphones on board of your planes?)
Answer: "Andrzej, według naszej informacji nie ma ku temu przeszkód." (As far as we know, no restrictions here).