Hard to believe but it was done heavily on flights until the 70's or 80's or so. Though disgusting, it is unlikely to be that dangerous.

Because there's a cost in attention. What's the rate of people just lighting up to smoke on a flight? What about the rate of people tampering with or disabling smoke detectors? Is there reason to believe that in the last, say, 5 years, there's been any case of that warning preventing someone from disabling a smoke detector?

I'm also not convinced that accidental smoking on board is that dangerous. Because if it were, then we'd be far more interested in stopping people from boarding with such equipment. I'm hoping aircraft today are capable of handling a small fire, the kind that might happen anyways, like from batteries exploding.

Same reasoning for using electronics - if they had been that dangerous, any bad person could just ship a box of phones with a wake up alarm set and watch planes crash.