People in LA need to breathe during the summer time. So yes we demand a blend that protects our residents. And the refiners are choosing to close refineries. They are not being compelled.
The improvement in air quality is due to the clean air act, catalytic converters, and the shuttering of industry, the gas blend plays a minor part. Even then, with gas so much higher it will materially make peoples lives worse, at some point society would be better off getting rid of the blend.
Whatever they're doing seems to be working nicely.
That ended a long time ago. A modern Honda generates something like 1% as much pollution as a car from the eighties.