Re: So they want..
With ipv6 they can do a 1-to-1 mapping of IP address to device, which surely is much stronger from the point of view of bureden of proof.
Unless the Person Of Interest uses temporary IPv6 addresses and/or any number of related wheezes.
But of course the whole discussion overlooks one important aspect.
DISCLAIMER: I have dual-stack service at home. My computers all have an RFC 1918 IPv4 assigned locally and hide behind the IPv4 on the "outside" of my router. They *also* have an IPv6 address "computed" by appending a machine-specific portion (EUI-64 normally, but it doesn't have to be) on the back of an ISP-supplied /56 prefix.
Conclusion: they all have the same public IPv4 address, and they all have the same IPv6 prefix.
Conclusion to the conclusion: if you're an alphabet soup looking to link together activity by my computers (desktop computer, network protocol analysis computer(1), dedicated video game computer, pocket computer, wrist computer, etc.) in IPv6, just look at the prefix. It's actually *more* linked to me than my public IPv4, since I can't change the prefix by rebooting my router.
(1) UTM-grade firewall. My company makes them, so I get to use one at home for free. It was amusing(2) to report a bug when it identified the connections to Final Fantasy XIV's gameplay servers as being a particular industrial networking protocol, and then shutting them down because they were, in fact, NOT that protocol.
(2) Amusing because it's not sold as a home-user product, so MMORPG connections aren't exactly normal for it, but now the automated regression test suite includes an FFXIV connection.