Beats me. I can't find anything definitive on this. Since Apple devices continue to broadcast Find My signals even when powered off (as long as they have a little bit of battery left), I assume they continue to do so in airplane mode.

It wouldn't do a lot of good if thieves could just turn off Bluetooth, right?

Airplane mode doesn't turn Bluetooth or Wifi off at all on recent OS versions of Android or iOS.

Really? What's airplane mode even for then?

It disables cell data and calling.

really? Why did they even leave the feature in then?

It turns off celular which is the main problem (Wifi is common enough in modern airplanes so I have to assume the interference risk is low, same for Bluetooth)