[cafebabbe]

Most adtech players are legit and don't rely on such practices, illegal in many countries.

Of course some smaller one will still try to do it, don't get me wrong.

[eivarv]

Well, there's the entire field of databroking (likely completely illegal under GDPR, not to mention unethical from the start) and real time bidding related to this.

Given the sorry state of privacy-compliance in general (whether that's ignorance of the law or consciously taking a risk) I seriously doubt that the field of adtech is much better.