Many private airplanes also do not have ADS-B.
Aircraft age and type: Older aircraft, especially those with simple systems, are less likely to be equipped with ADS-B.
Flight path: Aircraft flying at lower altitudes or in less-controlled...
Sorry, just found the time to respond. I checked the rootfs and saw USB/debug scripts (what I meant by debug mode) and a kernel console on ttyPA1, so I doubted they left no recovery path. Like you said, it's probably DFU over USB or an internal...