As is and would continue to be done innumerable times everyday by mechanics despite any lack of vehicle safety testing as has historically been required by the states. Personally, I cut back turning wrenches considerably in '76 and by '80 had discontinued the practice entirely (I still tinker) having landed an engineering position with a distributor of major heavy equipment and industrial engines.
Fuel additives and larger injectors can defeat the effectiveness of emission controls, not that they'll necessarily increase power.
Pull off any number (EGR, PCV, Sensor ...) of wires, hoses, or lines; one could also easily have multiple devices either fail or disabled (that don't prevent the engines from running) and significantly decrease the efficiency, and increase the pollution output, of the engine.
I'm only surprised at the length of your run-on sentence.
I worked tune-up and electrical in '74-'76 at a Mopar dealer. Remember the red, sometimes off white, idle mixture limiting, plastic stops that covered the screw heads on Carter's (which also had an issue with warping, requiring a retro-fit brace)? Periodic rough idle complaints on new cars were sometimes addressed by first subjecting such engines to a full Sun Scope (on a rail) diagnostic. Were no issues found, I would remove them, as emissions testing was neither available nor required. Never once had a comeback or complaint.
No they were not. "Cleaner air" evolved from unleaded fuel, catalytic converters, fuel injection, and overall drive train computer management of hundreds of millions, not the hobbyists' thousands, of vehicles on US roads.