I couldn't say how common it is, but military aircraft definitely fly in American airspace without transponders. A few months ago I hear/saw some Osprey fly ov

I couldn't say how common it is, but military aircraft definitely fly in American airspace without transponders. A few months ago I hear/saw some Osprey fly overhead. They were not visible on ADS-B Exchange (which does not block military transponders). What if it accidentally collided with a civil airliner? Not much. I can't think of anything more recent than Hughes Airwest 706 (and that predates mode s transponders by a few decades), and all that came out of that was a promise to communicate with civilian ATC.