I believe that MbTE over ethanol was a State choice?
https://archive.epa.gov/mtbe/web/html/gas.html
Could have been an economic one as having a "designer" fuel for just NH might have been problematic.
PFAS are also being regulated by each State. Some are quicker than others to place the limitations or restrictions.
And CB is definitely a State issue, as EPA run-off proposals are not popular with our Legislature.
OBD over tail pipe testing was a State choice, and expanding it across the State rather than only the areas that had Clean Air Act requirements again was the State.
Currently, we may still just have a few areas - if any - in the State that have problems meeting the standards. Which is why requesting a waiver would be smart regardless of the actual safety requirements that the Legislature may choose to reimpose.