I had a couple AC flights this summer. YYZ-FRA and LHR-YYZ. I was told the same thing on both. Makes sense from a safety standpoint. You are able to hear instructions during takeoff/landing which are the times when something is most likely to go wrong.

I was also told exLHR that I couldn't use my headphones in Bluetooth mode. Not a problem since it was plugged into AVOD.