There are lots of threads on this topic, with some saying it works and some saying it doesn't. I'm part of the latter. I put an AirTag inside my checked luggage, which works just fine, but tracking it throughout the airport didn't work. I mainly did it in case it got lost; but for kicks and giggles, I did open the Find My app occasionally just to see what's going on.
The 'last seen' location was pretty much where I dropped it off. It wasn't seen again until it was time to board the plane; but even then, it looked like it was on the plane at the next gate over. (Keep in mind, the location of the AirTag is actually the location of the phone it pinged off of, not the location of the AirTag itself.)
I suspect the checked bag went underground through their automated system after I dropped it off. Couple issues with this... 1) Probably not many people, if at all, down there for the tag to ping off of. 2) Even if there were people with iPhones down there, their phones probably didn't have a GPS signal.
I couldn't see the AirTag during the flight, which I expected. The location didn't update again until it came out on the baggage carousel to be picked up.