The low-compression engine in a 1966 Cessna 172 doesn't require leaded fuel; there are at least two supplemental type certificates available to legally run it on unleaded auto fuel. The lead is in the fuel as an octane booster, and is required by the high compression, higher horsepower engines much further up the food chain which are increasingly being replaced in the marketplace with Jet-A fueled turboprop engines (or even in some rare cases by Jet-A burning diesels).

Leaded 100LL avgas is certainly going away...

http://www.autofuelstc.com/approved_engines_airfames.phtml

https://swiftfuels.com/