I think there's some misunderstanding because there are two types of regulations at play. One type of regulations or standards is trying to increase efficiency of internal combustion engines so they use less fuel for the same amount of work (e.g. miles per gallon). This helps reduce the amount of carbon dioxide produced when burning fuel. Carbon dioxide is a greenhouse gas (not my opinion, it's just the official definition). Back in the day, the US administration "classified greenhouse gases as pollution that endangers public health" which meant enforcing emissions control fell under the EPA (just like cleaning up abandoned toxic waste sites).
The other type of regulation is reducing the pollutants like NOx, particulates, fuel vapors, etc. These are the things that cause smog, acid rain, etc. DEF doesn't affect carbon dioxide output, DEF only reduces the nitrogen oxides (NOx).
The current US administration effectively had carbon dioxide delisted as a pollutant so enforcing limits on carbon dioxide emissions is no longer under the EPA because it no longer endangers public health. That's really all the article is saying in the OP's first post.
So I don't think DEF or any of the current emissions stuff will fade anytime soon as all that stuff is really aimed at reducing all the other pollutants besides carbon dioxide. Engine manufacturers may receive relaxed limits in how they must increase "miles per gallon" for their engines but that's probably the only relaxation. And it was this 'increased mpg' reason why electric vehicles have been pushed so hard to be incorporated into fleets as the EVs made the total carbon dioxide emissions get averaged down to meet standard. Without the need to decrease carbon dioxide outputs, I doubt EVs, especially for heavy equipment, will make sense in the near future.
