Cookie banners were invented for this purpose but as we can see this is not the best solution. How would anyway anyone opt-in to any kind of tracking after all the privacy issues in the last years? But some kind of standard would be really important for sure. I'm working on the implementation of tracking solutions with the respect of user consent and privacy but even simple website analytics gets really complicated because there are no industry standards currently, every 3rd party handles user consent differently. And we did not even got to the point of gathering the user consent..