Comment 4 for bug 241305

You are aware, though, that IPv6-only hosts are still a rare exception
on the open internet and that you must expect not being able to connect
to all services - like Ubuntu updates.

I am not saying that it wouldn't be a good idea to have at least one
update server accessible via IPv6 but it is neither a bug nor a security

As a solution: Setup an apt mirror on a dual-stacked host in your
network. Look at the apt-mirror package for this purpose, it is quite
easy to do.