VST/AU have a very annoying technical limitation:
Both plugin architectures allow plugins to provide user interfaces for configuration, and apps that support plugins generically must display these interfaces. The way that the VST/AU folks chose to do this requires that the user interface be displayed in the same process where the plugin is loaded.
This is a big problem for Roon, since for many users, the server and the UI are running in different processes (RoonServer vs Roon) or different machines, and often not even on the same platform.
It’s frustrating that the vendors of these plugin systems didn’t think bigger, or consider that architectures other than “standalone app with UI” might exist one day.
Anyways, that’s why, as much as we’d like to do it, I don’t think VST/AU are in the near future for Roon.