This looks like an anti competitive action that should be investigated.
In other words, they have just closed down that entire ecosystem. I don't think there's an antitrust angle here: Forcing e.g. Apple to allow third-party apps or app stores seems within reach (at least in the EU); this would be more like forcing Apple to enable apps in a world where they don't even exist (anymore).
Here, "conversational" means your voice command should prompt the app to ask you a question, driving a state machine. (Set a reminder. When do you want to be reminded?)
I haven't tried to use it, but the docs indicate CRUD-over-voice ("built-in intents") is unaffected.
https://developers.google.com/assistant/ca-sunset#what_will_...
https://developer.android.com/reference/app-actions/built-in...