I have been around ever since 2014 and I honestly have never seen anybody in real life that accepts altcoins as payment. You may find some places here and there on the internet that accept altcoins through payment processors which would give them the fiat equivalent right away into their bank accounts but never in real life.

I've seen Tether be used for P2P trading too but that is not payment. For example I recently sold a tiny amount of bitcoin for Tether and then dumped it for fiat through P2P trading.