I mean, it is secret. Hardly anyone on HN knows about this let alone the average Chrome user. Google made the deliberate decision to not disclose this tracking

I mean, it is secret. Hardly anyone on HN knows about this let alone the average Chrome user. Google made the deliberate decision to not disclose this tracking, right? That's the definition of secret. I won't speak to legality since I'm not a lawyer, and I never claimed it was illegal for that reason. If this feature was disclosed to users with an opt-out, it'd be significantly less suspicious. Instead you've got a secret hard-coded mechanism for tracking users that is impossible to disable. Maybe the reason people are suspicious is because it's a little hard to trust "we don't use this for tracking! trust us!" from a company that makes their money from advertising and tracking (including shadow profiles). Additionally, Google could be compelled to use this data to track users and lie about it publicly through the use of National Security Letters or other nation state mechanisms.