The whole point of this decades-long unleaded AVGAS saga is to develop a fuel that is a drop-in replacement for the current leaded AVGAS. A fuel that needs to b

The whole point of this decades-long unleaded AVGAS saga is to develop a fuel that is a drop-in replacement for the current leaded AVGAS. A fuel that needs to be stored either in a pressure tank or chilled does not fulfill that requirement. Now, if you're developing a new plane from scratch, I guess propane in theory could be an option. But why? For aircraft, energy density of the fuel is important, and propane does pretty well here, slightly better in fact than gasoline or kerosene (jet fuel). But in practice more than offset by the requirement to store the fuel pressurized. And then you have the whole fuel infrastructure issue, how are you gonna convince airports to stock propane considering they're already struggling to find an economic case for AVGAS due to the long-term decline of general aviation. I think if you're going to develop a new plane, it would make sense to make it use the fuel that's available, namely Jet A1 (well, maybe electric for some usecases). Sure, it's not usable in an Otto engine, but since you're developing a new plane anyway, power it by a diesel engine or turbine.