Military aircraft have permission from the FAA to turn off one, or both, for fairly obvious reasons. https://nbaa.org/aircraft-operations/communications-navigati...
My dad's little four seat hobby plane has both In/Out. You can track him on FlightAware as a result, because it's continually broadcasting its location; it's certainly not rare or sophisticated equipment.
Here's a military Blackhawk toodling around as we speak: https://globe.adsbexchange.com/?icao=ae27fc
https://www.avweb.com/aviation-news/blackhawk-ads-b-was-off-...
> The Army Black Hawk helicopter crew involved in the midair collision with an American Eagle CRJ700 last January at Reagan National Airport had turned off ADS-B because they were practicing a classified flight profile, according to a New York Times investigation.
I have been running an ADS-B receiver at home for 6 years via PiAware along with an AIS receiver. So yes, low cost :)
https://download.aopa.org/advocacy/2019/dhowell_jking_DASC20...
> A majority of respondents had used ADS-B In, with 56% of respondents reported having experience with either an installed or portable system. Of the group who had experience with ADS-B In, 85% used portable systems and 30% used installed systems.
And that's in 2019.
1. C-17 Globemaster III (transport)
2. C-130 Hercules (transport)
3. KC-135 Stratotanker (tanker)
4. KC-10 Extender (tanker)
5. P-8 Poseidon (maritime patrol/reconnaissance)
6. E-3 Sentry (AWACS)
7. E-8 Joint STARS (reconnaissance)
^ above have ADS-B In capability
This answer on Aviation Stack Exchange did some research into ADS-B statistics for military aircraft: https://aviation.stackexchange.com/questions/107851/military...
TCAS (collision avoidance) can use Mode A/C/S however it depends on if the aircraft has the earlier or later model TCAS: https://aviation.stackexchange.com/questions/90356/does-tcas...