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.
Only sort of; the engines were designed so that the additives were important to combustion performance. Of course, the solution isn’t to keep using leaded fuel, but to rebuild the engine to be more reliable AND not depend on the additives.
Running an old engine with modern fuel will definitely cause it to fail.
It’s only important for increasing octane. You would have to reduce the performance of the engine on order to prevent knocking.