I actually wondered about this. My iPhone can be located even when turned off, so long as I haven't deactivated that functionality at shutdown (has to be done each time). Is an iPhone in airplane mode really less trackable than an iPhone that is turned off?
I feel like the BT behavior varies when I activate airplane mode. If I am currently connected to my BT headphones, they stay connected. If I am not, then activating airplane mode appears to turn off BT (pulling out my headphones and trying to connect doesn't work). But I would be surprised if the Find My functionality didn't work in airplane mode, given how Apple set it up to work even when turned off (you must manually set it not to be active, and you must put in your passcode each time).
When I turn on Airplane mode on my iPhone, it switches bluetooth into that "not quite off" mode that you get if you just tap the Bluetooth button in the control center (the icon goes white rather than a cross through it like if you turn it off completely in Settings). The Not Quite Off mode IIRC allows stuff like Apple Watch, Apple Pencil, Handoff, Find My, etc[0] to use Bluetooth but it disconnects all the paired Bluetooth devices you are using (basically it solves that "damn my phone is still connected to the headphones in the other room!" problem that 90% of people turning Bluetooth off are trying to fix)
