> For instance VST3 does not support many to one, and only awkwardly supports two to one -- via second-class "side-chaining" -- sound processing architectures

This is a limitation of your host and plugins and not of VST3, plugins can declare arbitrarily many input/output busses for audio and events for many-to-many connections. It's just that in practice, hosts don't like this, and JUCE has a busted interface for it.