I've split out the code that performs this check (based on an idea like old RepoPrune), added a brute-force check for dead.package files (via http and cgit), and the current working-copy is this: http://mschwendt.fedorapeople.org/obscheck-remote.py Output for Rawhide:
Dead and all builds obsoleted: ------------------------------ chktex ff-utils jadetex rdate
Undead and all builds obsoleted: -------------------------------- classads detex drupal6-drush gpp4 ibus-table-array30 jaxen-bootstrap joystick kdirstat latexdiff mate-conf mate-conf-editor metapost-metauml nss-myhostname pdfbook pdfjam ps2eps python-cryptsetup python-lvm python-quantumclient seahorse-plugins tetex-IEEEtran xmltex zeitgeist-datahub
Only obsolete subpackage(s): ---------------------------- lzma ['lzma'] 2 left openswan ['openswan'] 1 left ruby-gnome2 ['ruby-gtksourceview2', 'ruby-gtksourceview2-devel'] 24 left yum-utils ['yum-plugin-security'] 27 left