My understanding is that this doesn’t exclude you from the emissions laws. Obviously a Model T can’t be expected to pass a modern emissions test, but if your antique car had a catalytic converter it is technically illegal for you to remove it. The vehicle has to be equipped with whatever emissions it was built with.
Pennsylvania (where I live) has Classic plates and Antique plates. Classic plates are for vehicles more than 15 years old, and they have to have a yearly safety inspection, but they do not need a yearly emissions inspection (PA requires yearly safety and emissions inspections for all regular vehicles). Antique plates are for vehicles more than 25 years old, and antique vehicles do not need a safety inspection or an emissions inspection.
I bought my 74 Super Beetle several years ago. For as long as I have owned it, it has never been inspected by the state in any way, shape, or form. There are rules about how many miles I can drive it in a year, but no one has ever checked its mileage. The car is supposed to be “substantially as manufactured”, but no one has ever checked that either. Old Beetles don’t really have anything in the way of emissions controls on them, so it’s not an issue for me, but technically, if an antique car in PA was made with emissions controls on it, it’s illegal to remove them. From a practical point of view, I could do pretty much anything I want to this or any other antique vehicle and no one from the state or federal government would ever know anything about it.
I suspect that very few Classic and Antique vehicles in PA still have all of their emissions equipment installed.
Incidentally, PA’s emissions inspection is that they plug an OBD-II scanner into it and let that decide if it passes. If the vehicle is older than OBD-II was required (1997, I think) then all they do is a “visual inspection” where basically all they do is look underneath to make sure you haven’t ripped off the catalytic converter. It’s all of 30 seconds or so of work either way, and for that they ream you for $45, most of which goes to the state, not the mechanic.