Key things here:
The gas used in the small planes with piston-driven propellers has lead in it. Yeah, the same stuff that was banned from automotive fuel because it causes brain damage in kids.
That lead in the fuel airplanes burn? It ends up in kids blood
There’s a new fuel mix starting to be available which doesn’t contain lead, and which works for all airplanes. It’s available in some places in the US, but not all of them yet.
The FAA has a plan to make going lead-free a requirement by 2031…but right now there isn’t anything stopping airport owners (often local governments) from making the switch happen faster. Only thing which might stop this is that the Republican version of the FAA reauthorization bill contains a requirement that airports keep on selling leaded fuel forever, so it’s also important to stop that from becoming law.
Another issue is that so many small airplanes use engines designed in the 40s and 50s. Sure there have been some updates over the years but the basics havent changed because of the testing requirements from the FAA on new engine designs. Aviation need to become faster at bringing affordable updates to aircraft that dont also limit capability
That definitely made it a lot harder to end the use of lead. Those old engines account for about 1/3 of the small planes in use, and made it a lot harder to develop an unleaded aviation gas suitable for all planes. (There is a widely available one which can be used in the other 2/3 already)