What happened to gasoline in the mid 1970s?

You are in the ball park. THere was a long road of learning what worked, what didn't and how it all developed. The stories that I hear......
Air pollution controls started in the late 60s. PCV systems had no impact on performance but did help some. Air injection into the exhaust stream was common on GM cars starting in 1967 or 68. Ignition timing and leaner carburetor settings were implemented but the engines could be tweaked to run "right" by any gas station mechanic.
The Feds wanted even cleaner air and had several suggestions on how to get there but as with any Government program, they look years ahead to enact incremental changes.
Electronic ignition systems were developed and in place by most manufacturers by 1974. Catalytic converters were pretty much everywhere by 1975. A single pipe on 4 and 6 cylinder engines, dual pipes merged into Single exhaust on V8s.....through a restrictive converter. The converter had to be restrictive to slow the exhaust long enough to clean it up. A converter that flowed well didn't have enough impact on the emissions. The factories found that the exhaust had to be hotter to burn off more of the emissions so some cars were fitted with pumps that injected fresh air into the exhaust stream. This caused an chemical reaction, raising the exhaust temperatures.
Compression ratios were dropped to the high 7s to low 8s in the vain attempt to run without detonation on low octane 87 unleaded fuel. Don't be fooled though. Published compression ratios of the Muscle car era were bullshit. My own '70 383 was rated at 8.7 to 1 but calculated to 8.12. The ratings for the other engines are likely just as wrong.
Closed element air cleaners with the intake bypassed to a heat stove pipe attached to the exhaust manifold supposedly aided in faster warm up periods. All of these efforts worked fairly well when new but they didn't age well. Chrysler had their "Lean Burn" system that had a lot of tow truck companies rich!
What first looked like D O O M for the auto industry turned out to be the savior.....Onboard computers! The computer, along with EFI, made the cars run better, get better mileage, produce far fewer emissions and last a lot longer. Precise metering of the fuel and the constant ability to react to different conditions allowed the engines to make more power than ever. There is NO way that a Challenger Hellcat or Demon would have ever made 707 or 843 HP and pass emission testing without a computer.