I can add a little enlightenment here from Spain.
At the beginning of this year motorists were advised they should purchase an 'emission sticker' for €5.00, that would remind you when it was OK for you to drive into an emission controlled 'city', the rule is active but as of yet not being policed.
So all electric you get an 'A', hybrid gets you a 'B' and all but the least technical of cars gets a 'C', anything remotely old my 80 for example does not get a sticker at all!
At present the plan is simple enough, as you approach a 'city' if 'A' & 'B' is displayed on some new overhead sign, then only an 'A' & B' vehicle can enter, a 'C' or older vehicle can go elsewhere and find a parking spot and use a bus or walk in, I also see this as a covert measure to help push the purchase pf electric vehicles.
But there has been a change already for example, as there has been a lot of resentment about this being seen as another undercover tax and many refusing to pay it, the resentment also fuelled 'pun intended' about the fact that farmers who burn off fields with enough smoke to cause a 'city' to disappear in the smoke with no interest taken by the Government! But also most people who live and work in 'cities' tend to walk or use public transport, so the word 'city' no longer applies, the change in regulations is now decided by population. So any 'cities' and non 'cities' with a population over 50,000 has come under the emissions rules!
But there could be something of use here, non compliant vehicles my 80 for example 'may' be excused of the regulations under 'historical rules', there are not too many vehicles left being used on the streets every day that are over 25 years old, so their addition to the emissions is pretty much insignificant, so with this ammunition in hand, apparently historical clubs are taking this argument to the the government with a view to getting a free pass? Of course it would have to be present owners, or the second hand car market is going to boom!
EDIT: The above categories of A, B ect may be slightly wrong? I saw a vehicle in town as it went past and I swear it had an 'O' in the windscreen. I will keep an I out in the future.
Regards
Dave.