arches.desc specifies whether an architecture, as opposed to a profile, is to be considered stable. It does not replace profiles.desc, but supplements it; profiles.desc still describes the profiles of each architecture. https://www.gentoo.org/glep/glep-0072.html
repoman support has been removed per bug 835013. Please file a new bug (or, I suppose, reopen this one) if you feel this check is still applicable to pkgcheck and doesn't already exist.