Whether this is opt-in/opt-out is not clear. What bothers me, though, is how Google won't delete your images used for training automatically.

https://support.google.com/googlenest/answer/9320885?hl=en

> Note: Disabling Face Match will delete your face model from your device but it will not delete the enrollment images used to create your face model. To delete Face Match enrollment images, visit myactivity.google.com.

So, the real question is, why do we have to be aware of some gotcha that's deeply embedded somewhere in some support page that majority of us won't look at? This is the case with most of Google's opt-out strategies.