> The catch here is that OBS doesn't support a way to bus together multiple audio sources and apply plugins to their downmix, which limits the effectiveness of being a VST plugin host.
I'd not be shocked if a large part of this is UI/UX considerations. It's not like it is technically especially challenging to hand off a mixed audio stream to the VST plugin, rather than juts a source. But creating a UX that enables such things without being too confusing is not exactly trivial.
It is complicated enough that the OBS mixers is creating up to 6 output tracks. For twitch streamers, tracks 1 and 6 are typically full mixes, while 2-4 are often partially mixed tracks for local recording, to enable post processing (level adjustment, noise cleanup, etc) in the YouTube video creation workflow.
Enabling support for effects on one of the output tracks is probably feasible, but more likely you would be interested in applying affects to an intermediate mix, which can be used as one of the inputs for the final mixes.
And that starts to get hairy to create a good UI for, unless you just go for mimicking how a mixing console handles it. But many mixing consoles don't have the most new-user friendly user experiences.
And all of this comes on top of UI development work often just being a pain.