We don't know how to give users good anonymity when some relays can't connect to other relays. This would happen if we allowed IPv4-only relays and IPv6-only relays in the same network.
With "IPv6 only" relays available in the consensus the answer may be... when their count is the same as when IPv4 relays were at the same count, what was being stated and roughly understood about tor's anonymity back then? And is it much different from today. And given respective traffic loadings, etc.