No, not everywhere. Are you speaking about smoking or sitting in the cockpit? Before 9/11 the govt of the airline made the rules. The FAA didn't allow it so you would not see a passenger in a UA/AA/DL cockpit. But the UK's CAB did allow it (pilots discretion). A number of airlines allowed it and there was no law against it (that's changed for almost every country since 9/11

Same with smoking. It's not illegal to smoke on many carriers but it is illegal to tamper with a smoke detector or fail to follow flight attendant instructions. So if a flight attendant on say, SQ tells you it's ok to smoke somewhere, there is no LAW against it. It may be against company policy but you can't get arrested for that. I have NEVER EVER hid in a lav and had a cigarette but I will admit on some ultra long hauls I've asked the purser if I "could just have one". 9 out of 10 times the answer is no but 1 out of 10 I surprisingly get a yes. (Never on American carriers & only once on a European carrier) but sometimes on Asian or the ME3 they will find a place in the middle of the night to let me have a quick one...........it's not against the law on many carriers, just against the rules. But there is no rule that says you can't ask

It was no soooooo long ago that many airlines still had smoking sections. It went away on UA in 1997 but ANA, JAL, EK, and a few others had some smoking flights all the way thru 2000.