Most of the notices are implemented in such a way that the tracking is enabled by default and clicking "decline" in the notice sets a cookie saying "opt out" to all the trackers (whose effectiveness is probably equivalent to the "evil bit" in IPv4).

Blocking the notice (or ignoring it) is technically equivalent to opting in. Of course, if you're using a competent ad blocker it's likely that the trackers themselves were also blocked, making this a non-issue.

Isn’t that kind of implementation blatantly illegal, though?

It is, but:

* it's hard/impossible to prove - given how many factors go into ad targeting there isn't a conclusive way to prove whether an ad was targeted because of illicitly-collected data.

* as you see here even blatant GDPR breaches and acting in bad faith doesn't actually land you in trouble, so although it's "illegal", the law isn't enforced. Experian (or Equifax) got caught by the ICO knowingly misusing people's credit data for marketing purposes and all they got was a warning, so clearly the message is that "breaching the GDPR does pay".