Oh sure, let’s look at that new content. Retail is basically designed to funnel everyone into it by aggressively obsoleting everything else after all, so you’d expect the overwhelming majority of the active playerbase to be there.
I just logged into my Proudmoore toon. I can /who the full population of Nazjatar by filtering only by class, as none of the individual requests cap out. 110 people in total, with almost a third of them druids likely herb botting. This server is “Full”, I’m on the overwhelmingly dominant faction, and I’m polling one of the only two zones left in the game likely to be active.
Logged onto my classic Atiesh character. Same set of polls put Stranglethorn at 168. Almost every other zone of equivalent or lower level – and there’s a solid two dozen of those – shows similar or greater numbers. This server is “Medium”.