That’s optional, and opt-in -- it asks you during phone setup whether to enable Siri.
It does try to nudge you into enabling it, but if someone kept pressing that button and invoking Siri accidentally, they could open Settings and search for “siri” to disable it, which I think is reasonably discoverable.
It does try to nudge you into enabling it, but if someone kept pressing that button and invoking Siri accidentally, they could open Settings and search for “siri” to disable it, which I think is reasonably discoverable.