You don't have to have a subscription, but you lose event recordings if you don't have one. You'll still be able to view live video from a Roku device or the app on your phone. I'm not sure whether or not you get "motion detected" notifications without a subscription. Hopefully someone can let us know.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.