Quick note on this one. ETags have been used for years, but there is a court case from 2011 and before, speficaly noting this ETag technology as undeletable cookies. Was just a click away from the Wikipedia page. So I think anybody using it, would have an huge issue with the laws. Personally, I think the tracking pixels are more interesting to look at.
https://www.extremetech.com/internet/91966-aol-spotify-gigao...