Yeah, ammonia can also be used, it's, IIRC, more efficient to use it and a bit safer because you don't high pressure, buuut enough ammonia in the air will suffocate you.
It's lighter than air so it will go away by itself if allowed to. For a purely exterior instalation it's much better than anything else.
I have a heat pump that uses R290 (propane) (as well as an CO2 one for hot water) and would take that any day of the week over ammonia! The charge of propane is pretty tiny and the safety standards extremely strict, so not really concerning in terms of flammability, ammonia I would consider far, far more dangerous.
The “common” places I’ve seen ammonia refrigeration in use are hockey rinks and cold storage warehouses. I assume it’s used in industrial installations as well for heat rejection.
It's lighter than air so it will go away by itself if allowed to. For a purely exterior instalation it's much better than anything else.