The GDPR just missed a step. They should have mandated an automated way for users to present their preference. Like the old DNT flag for instance. With the legal framework behind it that would have made that flag actually useful and browsers would have brought it back quickly.
I assume this didn't happen due to industry lobbying.
AFAIK there was a legislative initiatives to do exactly that and the ad industry whined about it.
I think the result would have been similar to what happened when apple did it's Facebook nerf. Within the margin of error no one wants to be tracked and the ad industry knows this despite their fake "user-benefit" Spiel.
In the end it didn't happen and I can't recall what it was called.
I assume this didn't happen due to industry lobbying.