On IPv6, your OS should also enable the privacy extensions, so that your device has two IPs: a stable one for incoming, and a randomly changing IP for web browsing. Sure, it's the same subnet, but it would be silly to rely on this considering the many other ways we can track users.
It wouldn't surprise me if there are already databases which map IPv6 subnets to real names, addresses, banking data, ...
And anyone could just use that database or contribute to it.
And then jurisdictions such as the EU, Canada and California would consider the IP address to be PII, and it would be illegal to contribute to such a database.
Again, there are much more easier ways to track people on the Internet.
> You would probably have the same level of reliability with an IPv4 database currently
That is... a lie. The selling point of today's internet is that you are anonymous.
Also I don't know which "easier ways to track people" you mean.
No, it doesn't. Your ISP is the one who can take that decision away from you. I have Google Fiber, and my public IPv4 address has not changed in around six months, while my IPv6 block has changed twice in that same time. This is despite replacing my router and several multi-hour power outages. I believe the only reliable way to get a new IPv4 address is to call support.
Most server software cannot properly handle blocking of increasing IPv6 subnets.
And not only that, but my ISP assigns the same /64 subnet to me for months. Who needs cookies anymore if you can just track the /64? Even unplugging the router for a day won't assign a different prefix for me.