IPv6 enables insane precission of device tracking. A side effect of not having enough ipv4 addresses and using NAT is that all your home devices let's say, go out on the internet using only a single IP address. Also, the geo-location accuracy using ipv4 is at the level of a city. With ipv6 you can get to a geolocation accuracy of a building block and confidently target individual devices within ones home network since they will use unique ipv6 addresses.
Surely, privacy minded people will find a way around those, but out of the box ipv6 is a privacy regression for the reasons listed above