It’s worse than “allowed” to use, in the US it’s more like “required to use”. Newer engines that work on unleaded gas exist, and even some older engines are perfectly capable on unleaded gas. However, the FAA has to test and approve each part that goes on an airplane, and it’s a ton of work to do that for every type of airplane engine. Unless the government subsidized such testing and paid for unleaded fuel tanks to be installed all over the country, it’s unlikely to change anytime soon.
It’s worse than “allowed” to use, in the US it’s more like “required to use”. Newer engines that work on unleaded gas exist, and even some older engines are perfectly capable on unleaded gas. However, the FAA has to test and approve each part that goes on an airplane, and it’s a ton of work to do that for every type of airplane engine. Unless the government subsidized such testing and paid for unleaded fuel tanks to be installed all over the country, it’s unlikely to change anytime soon.