Right now, repoman uses 'special herds' magic to support 'no-herd' value for herd. As we're removing that magic, and there will be no more 'special herds', that magic should be removed and repoman should start complaining about 'no-herd' once again. However, if possible, it'd be great if it could fix metadata.xml automatically. This could be either done through 'repoman fix', or maybe even by 'repoman commit'.
I think a warning message would be good enough. Automatic removal seems like overkill.
http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=f4aaee6edd75
This is fixed in 2.1.10.20 and 2.2.0_alpha60.