Client side data is only part of the problem, and it's a common myopic argument to GDPR for opponents of GDPR to say people should just use their browser's cookie controls. Server side fingerprinting is as much as a problem, something the ad industry has adopted since Safari added ITP and are accelerating since Chrome has been making noises about following Safari and Firefox for a few years now. I suspect as soon as Google figures out how to do it without disrupting their own ad business it'll come into effect.