I believe it was switching over to Metal that caused this. If people have bought it before, they can still download an earlier version of it, but my guess is there’s only so many hours in the day and Taylor doesn’t want to spend time that he could spend developing Audulus 4 and other projects making it so that people who are held back by software made by other devs who can’t keep up with OS developments can download Audulus for the first time. Make sense? I mean if it were as easy as just pushing a few buttons or copy/pasting some code, or writing a few lines here and there, he’d do it, right? It might close you off Audulus for the moment, but how many months or years will you spend not using Mojave?
It’s not always this way, but it’s happened a few times recently - the Metal update and when Files came out for iOS and we relied on its functionality for the library organization. I mean, Audulus 2 still runs on Mojave! My guess is it’s leveled out and from now on you won’t need to keep updating your OS to keep Audulus running.