TL;DR: All source mirrors need to confirm/update the distfile fetch URL
for mirroring, to rsync://masterdistfiles.gentoo.org/gentoo/
Recently the USSG group at Indiana University discontinued their rsync
mirror, they have provided service for many years, but had to
discontinue it. Their mirror was specifically the initial mirror covered
in the Source mirror documentation, and then as mirrors joined, you were
provided with the private URL & credentials.
The Gentoo mirrorstats page shows that multiple mirrors stopped updating
around the same time (2020/01/22). I count 7 distinct mirrors right now,
and I'll be further following up with them.
OSUOSL has adjusted configuration on their mirror to no longer require
authentication, and the documentation has been updated to reflect that,
however now we need all mirrors to ensure their configuration is