This just happened to me while I was traveling. I am renting a car, and use Apple CarPlay to listen to my downloaded music on my iPhone. When I asked Siri to play one of the albums, it incorrectly chose a song I don’t have and automatically subscribed me to Apple Music 7-day preview. I was never asked if I wanted to join, and the only email I got said “Your Apple Music Voice Plan is now live. With your 7-day preview, you can play any song in the catalog with Siri.”
No way to unsubscribe, either. I’ve never had it where I was automatically subscribed to something I didn’t explicitly request. This is so wrong.