According to [1] it’s not just the military, but “U.S. federal, state and local government aircraft performing sensitive operations”

https://www.aviationtoday.com/2019/07/23/new-rule-allows-mil...

Correct, GP asked about military so I mentioned that it's legal for them to turn it off. It's also legal for intelligence, law enforcement, etc -- anything government-related with a good reason pretty much.

Phantom planes are also technically possible. One could even do it themselves using the "Havok" firmware for HackRF One. I won't link to it, but to quote its wiki, "Yes, it works. For real. If you want to try it out, DO NOT transmit on restricted bands and ESPECIALLY NOT on 1090MHz."

Yes, 1090MHz is probably the stupidest bad to illegally transmit on.

Not just because of the potential safety issue, but because of how easy it is to track you.

In many areas, they have Wide Area Multilateration equipment installed that will show the real location of the transmitter, instead of the fake GPS coordinates you transmit. Normally it's there to double-check the ADS-B coordinates are correct, to track older planes without ADS-B, and to give true altitude readings, but it will track illegal transmitters as a side effect.

Even in areas where they haven't installed a permanent MLAT system, they usually have some sort of equipment that can track down a 1090MHz transponder. For-example, search and rescue planes often carry one.

Alternatively, you could use computer vision and only record the ones not transmitting - that would be an interesting data set!