these days it's really not much different from IPv4: During the lifetime of a connection, the prefix stays the same, so that's equivalent to the IPv4 address before that.

The actual machine address rotates very often, so there's no real value in using this for identifying unique devices.

If you want to profile specific devices, you're much better off using the same attributes you were using with IPv4 (user agent, TTL, other protocol specific fingerprint techniques)

The only thing that stays static across connections is the provider assigned prefix and that’s equivalent to your dynamic ipv4 address.

At the least, they should push a null default route to users that connect (assuming we're talking about the kind of VPN services that advertise as "protect your privacy with a VPN!").

But crappy ones don't. And some people end up using crappy ones, because they don't know any better :(

Or rather: Using IPv4 doesn't guarantee non-crapyness of a VPN provider.

But: Working IPv6 support guarantees at least some level of proficiency by the VPN provider, so they might be more reliable candidates to begin with.

But there's more needed with IPv6 than routing properly. The VPN provider needs to assign IPv6 addresses to customers, and that's harder than just NATing stuff. It's almost like being an IPv6 ISP.

But I've done a toy implementation. To get "anonymous" IPv6 addresses, so I could test VPN service clients for IPv6 leaks, without pwning myself. I needed a little help from an IVPN engineer, but it wasn't that hard.