And you're also claiming every single router I'm going to encounter today has already set up IPv6 correctly as needed to mirror the security and privacy characteristics of IPv4 like this?
What happened was I vividly recalled seeing "stateless" as the default option for the router I had in mind over a year ago. Which was indeed correct. However, in response to the other comment I went back and checked again, and just noticed that "stateless" was in fact under the NAT page, not the firewall. The IPv6 firewall page is rather hidden so it's something I forget about completely, as I don't use IPv6. Looking there, the firewall page indeed does say it blocks unrelated input traffic by default.
This wasn't the entire story though, because I could also remember that ip6tables -L was empty when I checked it. I now see, though, that this must have been because when I had done this check, IPv6 was turned off, and that made the router purge all the rules. I had always assumed it was never setting them up in the first place.
Frankly as of right now I can't really verify what actually happens because I don't immediately get an IPv6 address when I enable it. I trust it does what you say. That said, I'm still not sure how I could rely on this fact being true everywhere. With IPv4 it's pretty much a given that you will hit a NAT due to the lack of enough IPv4 addresses, but with IPv6 they could easily assign you an IP address, so what guarantee is there that the router has things set up this way? It still seems like a risk vs. no-risk, with the same resulting decision.
What do you see?