Great, we're in agreement that self contained appliances can all use propane and isobutane then. That covers refrigerators, freezers, heat pump waters heaters (not split ones though, heat pump dryers, and a bunch of commercial refrigeration products like display cases at grocery stores. That was a good chunk of my comment.
What were seeing in e.g. Europe is monoblock heat pumps, where it's also self contained and not split and they use e.g. a glycol mix as to transfer heat between inside/outside. This has pros and cons of course, but it solves the propane danger for a split system with a larger charge by keeping the propane outside. The con is freezing climates where there is risk of pipes bursting during power outages, but that's manageable with failsafes. For true split systems, I agree that propane is problematic and CO2 is more promising. But we don't have to use split systems everywhere either. I'm not aware of any reason we can't add mercaptan to propane, unless it messes with the refrigerant characteristics. The low psi methane pipes can also easily make a bomb out of your house because the supply is unlimited, so I don't see that as more dangerous than a fixed (low) charge in a sealed system.
We can solve these issues though, and my point is that the work involved is worth it to eliminate the dangers of synthetic refrigerants. I appreciate your perspective, thanks for the conversation!