The always-listening portion is not. Also “enabled by default” still requires active user consent and agreement to plain English privacy terms. It is a step that cannot be missed when setting up the phone.
"Voice Match", the always-listening functionality of Google Assistant, also needs to be enabled. It requires training to your particular voice in order to make it more difficult for someone else to try to access your information using the feature. When being enabled a warning regarding the potential security implications of doing so is presented.