Another way to think of it;
Ok, so a car has an OEM exhaust from new, you drive said car for 5-6 years and it starts to devolp an exhaust leak.
So you take the car to an exhaust shop and simplly say fix it, so they then find there is a rust hole in the muffler and say no worries we will put a new muffler on.
If one were to need a mod plate, technically this person above would require one too. As no exhaust shop puts in OEM parts even when simplly replacing a dodgey muffler.
Hence requiring a mod plate for an exhaust is not true, as almost every car thats more than 10 years old would have had some form of exhaust mod made, and if a car were 15+ years old the entire exhaust would have probablly been replaced at some stage.