The original sort (and ping, and traceroute) could just have been updated to handle the newer format, rather than creating new utilities, was my non-sarcastic point. :)
Probably better in this case would be to have a small program that expands IPv6 addresses that you could then pipe into sort. That would be more in-keeping with the Unix Way™.
I went and wrote this and then realized you'd need two utilities. One to expand the addresses, and one to minify the addresses, so your pipeline would be:
cat ipv6-addrs.txt | ipv6expand | sort -V | ipv6minify
vs cat ipv6-addrs.txt | sort -6
The unix philosphy is do one thing and do it well. Ideally sort does sorting very well, including sorting of ipv6 addresses. Ergo, sort should handle ipv6 address sorting.