I’m not an expert on the finer points of ARA, but my understanding is that ARA doesn’t make much sense for Dorico because Dorico is not fundamentally an audio editor, which is really what ARA is all about.
ARA does allow the exchange of things like the tempo map between the host and the plug-in, but Dorico and the audio engine can already do that to a limited extent anyway, and we will continue to build that out rather than relying on ARA to complete the support for that.