Some fantastic, albeit unsurprising research from Marko Saric on how many website visitors will give a GDPR compliant level of consent to being tracked by a marketing analytics tool.
If you implement a proper GDPR consent banner, a vast majority of visitors will most probably decline to give you consent. 91% to be exact out of 19,000 visitors in my study.
That's a marked difference from the > 90% opt-in rate claimed by Quantcast. Admittedly, most implementations of their tool are themselves, falling short of being compliant with the regulations or ignoring existing guidance from the ICO.