Airplane mode usually leaves Bluetooth on, and AirTags communicate with iDevices via Bluetooth. It's possible a signal could get through all the bags, the aircraft frame, and to your (or another passenger's) phone.

I actually think luggage tracking is one of the less useful applications for AirTags because, in the US at least, all the big airlines' apps seem to have a way to show you your luggage tracking activity including when it was scanned onto the plane.