The Ubuntu IPv6 BitTorrent tracker (http://ipv6.torrent.ubuntu.com:6969/announce) does not track any releases newer than Ubuntu 12.10. When trying to announce e.g. ubuntu-13.10-desktop-amd64.iso, the tracker response is:
Failure: Requested download is not authorized for use with this tracker.
Note, that the IPv6 announce URL is included in the 13.10 torrent file. Accordingly, http://ipv6.torrent.ubuntu.com/ doesn't list anything newer than Ubuntu 12.10. (note: the link works under IPv6 only)