After nearly a lengthy period of time being broken, then being officially unsupported by Google, Android Auto is getting back a useful feature. Google Assistant routines are finally working on Android Auto again.|
Presumably through a server-side update or through the Android Auto 6.1 app update, Google has reinstated the ability to run Assistant routines from your car. Since October of 2018, Android Auto users have been lacking this functionality despite having access to it prior. In early 2020, Google came out to directly say that the feature was not supported by Android Auto, officially.
Now, the functionality is back and working like a charm. While testing out the newly added wallpapers this morning, I was able to run a custom routine on my account that launches the robot vacuum cleaners in my home, with the screenshot below showing Assistant starting up a Shark IQ vacuum.
From what I can tell, routines on Android Auto work just like they do on any other Google Assistant device. The Assistant box stays up the entire time the routine is running with minimal feedback unless the routine demands it.