in Germany we also have restrictions.
you apply for a green sticker (euro4), no problem say on my spider (1990) which has a cat.
Historic cars (over 30 years and older) do not need a green sticker and can drive anywhere. Historic cars in Germany have an "H" at the end of the registration, so officials can easily tell.
foreign visitors to Germany "from any country" also need this sticker (5 euros one time payment) and it can be got online if you send them a copy (pdf) of your car registration papers (you could
otherwise get a 40 euro fine.......personally I've driven here for years in english cars w/o this sticker and no-one ever cared tbh.)
Foreign visitors in their 'historic' cars just need to lay a copy of their registration in the windscreen, so that if parked, it can be seen as "historic"
But true, cars that fall between Historic (30 years old) and non cat cars, will also have a problem here entering the green zones (but, only in the major cities!)....or you fit an aftermarket cat..of course you can drive in any car anywhere else all around Germany and Europe.....if they banned pre 1995 cars during the week outside the historic cities in Italy, there'd only be a few donkeys and chickens on the road, methinks