The Snapshots repository is now being used to archive unsupported ROS distributions which are about to be removed from packages.ros.org (see Security issue on ROS build farm for details) using a datestamp of final.
In order to allow for future snapshots to include repositories beyond just Ubuntu or Debian repositories, we’ve also moved the apt repositories into an ubuntu subdirectory symlinking to debian when appropriate. The earlier snapshots are available both at the datestamp root and the ubuntu subidrectory but future snapshots will be published only to the ubuntu subdirectory.