What happened to gasoline in the mid 1970s?

The simple answer is the gobt/ EPA wanted lead out of gasoline. Lead containing chemicals in gas was THE octane 'thing' back in the day. tetra ethyl lead, also back in the day called "ethyl" meaning you wanted the high octane stuff instead of "regular."

And of course emissions controls in engines BEFORE decent EFI just 'finished things off" Engines were lower and lower compression, leaned way out, retarded timing, ran hotter, less power, "afterrun" also called "diesling" IT WAS AWFUL

Emissions controls were several and combined. Lean out the carb A LOT, retard the timing and slow down the advance curve, lower compression to use lower octane fuels, and in the case of most all GM and Ford use "smog pumps" which pumped air into the exhaust manifolds to "burn off" leftover chemicals. In later years, catalitic converters were added to the exhaust. Exhaust recirculation EGR to reduce part of emissions. And various stop gap controls on the engine

Some wiki on "ethyl"

Tetraethyllead - Wikipedia