well... First things first.
if it’s just a cat back, about the only thing you really affect is sound. However... You are tampering with the emissions system. Should you have an emissions related failure; that is, a component of the vehicle that is explicitly called out in the manual as an emissions component, yeah... they could legally tell you to fix it yourself on your dime.
Now, I am not aware of anyone having trouble with a power train, electronics or other system not covered as a result of a simple cast back or similar exhaust. Not to say it hasn’t happened, just that I haven’t heard of it.
What if you suffer a busted catalytic converter at some point, can they void your emissions warranty and not cover the cat under warranty? That seems to be a stretch to me, but who really knows.
In all likelihood, it’s a pretty clear situation that mufflers downstream of the cats aren’t causing an emissions failure. I can’t envision a mechanic or service advisor trying that one out. it’s just... kooky.
