[noipv6]

6to4 has been de-preferenced by most operating systems.

what isps provide 6to4 tunnels, or even 6in4 tunnels (which i can only assume you meant?)

(agreed on the proxying point, you'd have to explicitly trust the proxy operator)

[tlb]

It's de-preferenced relative to v4. Which, if you have v4, you should just use directly.

I was wrong above -- NAT64+DNS64 (not a tunnel) is generally what v6-only ISPs provide.