"Hey Siri" has in fact been tied to the owner's voice since the iPhone 6s (2015): https://www.macrumors.com/2015/09/11/apples-hey-siri-feature...
The only problem here is that Google still doesn't recognise my voice half the time, perhaps because I'm never sure about how I should speak to an inanimate object.
Speaking of unlocking methods, face unlock appears to no longer be available as an option for me. I can't find it in settings anymore (smart unlock etc).