Just came across this issue when I added a herd to herds.xml via api.gentoo.org. Afterward, repoman full failed due to unknown herd. Just as repoman grabs the metadata.dtd, it would be nice if repoman could check the online, canonical copy. Suggested by _robbat2 in #gentoo-infra.
Well this 'fares well' for the case of dropping the concept of herd in the modern gentoo. Yet another background step to further entrench and tangle the use of herd as an essential key step in standard portage processes / protocol. But was that debate resolved in the negative?
herds.xml no longer exists.