Which is the best selfish reason to remember to switch on flight mode on an aeroplane. (Also when you’re trekking in areas where you know cell coverage is nonexistent.)

A literal example of skin effect?

Could probably gives non-permanent tattoos with this method.

If the phone can't hear a network (not just faint signal like airplanes on certain heights), would it actually transmit?

I think the battery drain would be from active RX searching for networks instead of sleeping. You get a similar though probably lower effect, when in roaming without setting a fixed network - an idle phone without crapware will burn way more energy.