There was a similar law being considered here in Arizona about 16 years ago, all vehicles over 20 years old would be taken off of the road, for the same environmental concerns.
Old vehicle collectors clubs rose to the occasion, and the law went away, and stricter emissions testing was implemented as a result.
Maricopa county the county where Phoenix is located, requires motorcycles to be emissions tested every year.
Only county in the US to do so, even the state of California doesn't require motorcycles to be tested, unless this has been changed very recently, and I'm not aware of it.
The Arizona legislature passed a law in 2008, exempting vehicles that have collector license plates ( 25 + years old), and collectible auto insurance, to be exempt for emissions testing, due to the small amount of roadway use these vehicles have.
I have two vehicles that are exempt, the '81 R65, and '79 Trans Am.
The emissions tests were more costly than the registration was for one year !!!!