I therefore do _not_ want to disable IPv6 entirely, but I still do
want apt to do its download over the [...] IPv4 link.
has become a much more common situation than when this wishlist bug was initially filed. (For corroboration, grep for ipv6 and ipv6 addresses in a couple days's worth of #debian logs.) It will become even more common as the ISPs and private networks of the world gradually meander through their 4-6 transitions. Wget has -6 and -4 options, and so does curl, after a fashion. Can apt?