I can’t know for sure what the parent comment was alluding to, but I would assume the idea is that because IPv6 isn’t built around an assumption of NAT, that it’s less private because you don’t have an intermediary router (potentially) concealing traffic origins as a network design feature.
That’s the only thing about IPv6 that I’ve heard of that I think is a reasonable argument regarding a degradation in privacy.
With that said, NAT was a fix for issues that weren’t about privacy, and NAT itself was not designed to give you privacy. It used to be that IPv4 connections were always direct in the same way IPv6 connections were designed to be direct.
In that regard you could make the argument that IPv6 is a regression, but that regression is effectively answering for an architectural shortcoming in IPv4 that we’ve tried hard to fix but that can only go so far.
Edit to add: NAT also doesn’t protect anyone’s privacy anymore than their ISP is willing to protect it anyway, for what it’s worth.
That’s the only thing about IPv6 that I’ve heard of that I think is a reasonable argument regarding a degradation in privacy.
With that said, NAT was a fix for issues that weren’t about privacy, and NAT itself was not designed to give you privacy. It used to be that IPv4 connections were always direct in the same way IPv6 connections were designed to be direct.
In that regard you could make the argument that IPv6 is a regression, but that regression is effectively answering for an architectural shortcoming in IPv4 that we’ve tried hard to fix but that can only go so far.
Edit to add: NAT also doesn’t protect anyone’s privacy anymore than their ISP is willing to protect it anyway, for what it’s worth.