Old.reddit is reddit from a time when it was designed with user experience in mind, rather than trend chasing and maximizing ad placement.

I've heard that the reason old.reddit is still supported is because new reddit can't run without it. I certainly wouldn't be surprised if the developers of new reddit were pushed to rush something out to meet demands from higher up, and therefore didn't make something clean and severable. I mean, we're talking about a site whose video player wants to load every resolution at once on every video you scroll past in an infinite feed, my expectations aren't terribly high.