In message <20170515124359.a3o7evaostrvm3tk@nic.fr>, Stephane Bortzmeyer writes :
Unfortunately, every time we've looked at the data, the conclusion has been that it would cause unwarranted user impact. IIRC the most recent blocker was a major US ISP whose clients would experience breakage if even just one NS record was dual-stacked.
There are many zones (including your isc.org) that have several name servers dual-stacked, and they didn't notice a problem. Furthermore, since the DNS is a tree, resolution of google.com requires a proper resolution of the root and .com, both having IPv6 name servers.
So, this answer is at least insufficient.
It wouldn't suprise me if the dispute between Google and Cogent was not part of the issue. Pure speculation on my part. I could be completely off base. Mark -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: marka@isc.org