Unlike "completely public spaces", it's not free to be on an airplane. You can vote with your wallet in this situation.
Since members of the general public are in close proximity I generally wouldn't consider an airplane cabin a place where I could have a reasonable expectation of privacy.
In places that are intentionally restricted from view by others, like my own home, a hotel room, etc. I'd have an expectation of privacy.
If I'm going to, say, a concert, or especially a taping of a TV show, I have a much higher expectation of ending up in some video footage. But people often sleep on planes. They're not expecting to be recorded.
Privacy is something I control by not exposing sensitive information in areas where recording is a possibility.
I already take precautions when flying by not working on my laptop or having sensitive conversations with family or coworkers.
I generally don't consider information on where my gaze is looking, facial expressions, or if I'm awake or asleep sensitive information unless I'm in a private space like my home, a hotel room, or a bathroom.
But aside from that, I'd posit that it's actually a very different thing to have a random passenger recording you (which is still gross) and the airline (which may be owned and operated by a foreign government, which is the case for most of the airlines in the Middle East and some in Asia), which has your name, your passport number, your birthdate, your address, and other very sensitive and identifying information on file.
One is creepy and uncool. The other could be flat-out surveillance. Especially since when you travel, the government data you give to the airlines isn't negotiable (meaning unless I'm flying domestic, I can't use a fake ID and name to travel (and even that would be against the law), I have to use my passport, my full name, my address, etc.
I'm not sure if they'd be able to extend that to someone recording me although it likely varies by country. I think British law considers continued recording a form of harassment where the same protections don't exist in America.
If I was really really bothered by it for some reason I might raise it once. The cabin crew has the safety of the flight and other responsibilities to be concerned with.
The problem is that this attitude will ultimately mean that the only place where you might have any chance of privacy is in your own home. And even there, privacy is getting more and more rare.
I fly a lot for work (I'm already at 67k miles flown for 2019 -- that's actual miles not airline miles (so that 67k is lower than my status miles)) and for anything over 6 hours or so, I fly business (unless I get an upgrade on a short-haul) and have had 6 flights over 10 hours this year, where I tend to have a lay flat bed and attempt to sleep. So I DO sleep on flights. Quite a bit (my flight to/from Australia was great, largely because I slept 9 of the 14 hours the first leg and 7 of the 13 hours the second).
I'd be just as freaked out if someone was recording me sleeping on an airplane in a ticket I paid $10k for as if I was when a hotel employee knocked once and then barged into my room in Sao Paulo back in December while I was changing into my pajamas. (And I was plenty mad at the WTC Sheraton for that!)
However my experience stems from two long haul flights in economy and a handful of domestic flights in the same class. If someone was recording me while I was sleeping there I'd treat it the same as if I was napping under a tree in a national park. I wouldn't be concerned.
If there aren't measures in place to prevent other members of the public from observing then I expect recording is a possibility.
In many business class seats, they are designed so that you cannot even see the person adjacent to you unless you specifically put your head out of the curved herringbone shell -- and that's when you're sitting. If you're laying flat, your head is completely obscured by the seat.
Delta and Qatar have doors on some of their business class seats.
People walking through the cabin can observe people sleeping, of course -- but these products are literally designed for sleeping and privacy. It's one of the reasons they charge such a premium for those tickets.