The only reason CFCs exist is because the US established a de-facto ban on propane as a refrigerant. You can't trademark, copyright it, or any other form of IP.
How is that a de facto ban, you can't trademark the common names of plenty of substances ubiquitous in daily life. The manufacturers of refrigerants don't seem like they could completely dictate what appliance manufacturers choose to use in their products.
It seems for refrigerators there are actually specific carve outs in EPA venting regulations to allow R-290 (propane) along with a couple others.
If vendors don't use it, possibly due to flammability, whether a real risk or as imagined by consumers that's a completely different situation. I can't find anything claiming it's banned or even discouraged by the government.
There are commonly sold flammable refrigerant things now in the USA, I see a bunch of small fridges using alkanes and cycloalkanes, not sure about HVAC.
Yes, if you release propane into an enclosed space like a mechanical room and then introduce a spark, it could cause an explosion.
The explosion risk (in a commercial setting) is mitigated by having gas detection sensors and an exhaust fan that runs when the gas detection alarm trips along with horn/strobes at any entrance to the mechanical room and inside the mechanical room.
In a residential setting, I’d only feel safe if the condenser (outdoor unit) used propane with a liquid to liquid heat exchanger and separate glycol loop that runs inside to the air handler coil.
