Do you have first hand experience with this in your line of work? I’m genuinely curious, I’m not in the industry but I have been pretty active in various car forums over the years. While I have personally only tuned one of my vehicles, I have read a lot of information across various platforms and I can’t recall any concerns or problems people have had with emissions and tuning only, though most of my vehicles have been newer and Illinois only requires emissions testing in certain areas and vehicles that are 3 years or older I believe. The only time I have seen people referencing needing to go back to the stock tune is relating to bringing the car into the dealer for any type of warranty work (though that may not work with key cycle counts), but I’ve never read anything about a tune only impacting emissions.
California has been collecting ECU data for years in preparation for checking for tunes. In the last year or two they implemented the program and now scan the ECU specifically looking for tunes during emissions testing. My guess is this was in response to diesel trucks rolling coal. For now, I believe only CA checks for tunes.
CA and EPA have also shut down and fined most of the Diesel tuners - including demanding and receiving customer lists (though I am not aware of them doing anything to customers yet). A few have stepped up to perform the drive cycle tests to sell their (now emission compliant) tunes, or folks are getting tunes from Canada or other countries.
It has always been illegal to tune any car with an emissions label (so every car that was not designed as a race car). There has never been an exception for modifying federally certified cars for racing / off road use, but until the Biden administration the EPA turned a blind eye. In the last few years the EPA and CA have cracked down on both the tuners and end users. I suspect enforcement will only get tougher. Eventually tuners for gasoline-powered cars will likely go underground or become emissions compliant just like Diesel has in the last couple of years
Green Diesel Engineering, LLC Company Case Settles for $50,000 In October 2018, Green Diesel Engineering, LLC (GDE), of Commerce Township, Michigan, settled its case with the California Air Resources Board (CARB) for emission violations related to sale of aftermarket electronic control units...
ww2.arb.ca.gov
Edit: I just read through the EPA settlement (last link) and it looks like the EPA crack down started during the Trump administration
"33. Respondents violated Section 203(a)(3)(A) of the CAA, 42 U.S.C. §
7522(a)(3)(A), by knowingly removing and/or rendering inoperative emissions-related elements
of the ECM installed on or in motor vehicles or motor vehicle engines that were in compliance
with Title II of the CAA, including the Black Ram 1500 inspected by EPA on July 9, 2019.
Civil Penalty
34. Based on analysis of the factors specified in Section 205(c) of the CAA,
42 U.S.C. § 7524(c), the facts of this case, information that Respondents provided to EPA, and
Respondents’ ability to pay, Complainant has determined that an appropriate civil penalty to
settle this action is $350,000. EPA has reduced the civil penalty based on information provided
by Respondents to support their claims that they are unable to pay a higher civil penalty and
remain in business."