Right now 66 of our 116 enabled mirrors have at least one AAAA record,
and the distribution varies between 40 and 60% between regions, so it
should be doable to exclude v4-only mirrors for v6-connected peers.
It's not always been the case.
However, my gut feeling is that most v6-connected peers are dual-stack,
and so far no one has reported this as a real issue. Personally I think
it's more useful to do that for TLS connections (redirecting TLS-connected
client to https:// mirrors). That's upstream issue #143 :-) There are
still quite a few countries without any TLS-capable mirror though.