Alas, that is not true. Yes, 99.99% of all the fasteners on a current vehicle are metric. Modern vehicles don't use muffler clamps, aka exhaust clamps.
Well, I don't mean to get into an argument about this (and this will be my last post on the subject), but having worked for one of "the big three", I can tell you that while you are correct that "modern" vehicles don't use "muffler clamps", they didn't even back when I worked in the industry (that is irrelevant to the topic). The whole purpose of the "EXHAUST" clamp was for repairs. And they ARE available in metric sizes (although truth be told, my honest opinion is that the metric and imperial varieties are essentially the same, and expected to deform to take the shape of whatever they are being clamped on - In fact, that would probably be my first choice to pursue if I were in this situation).
If you are referring to the actual nuts that do the tightening, then imperial or metric is of no concern, other than that the installer has the correct installation tool (and even then, most of the common sizes can be "faked" with the opposite systems "sorta-kinda" equivalent).
--Roy K3RLD (one time brake engineer, race mechanic, racecar builder/owner/driver)