The old-releases.ubuntu.com mirror hosts the repositories for EOL Ubuntu releases. The main page currently lists up to 11.10 (or 12.04.3 if you count point upgrades), but the dists page lists up to 13.04. Why the discrepancy? When will the main page be updated? When will the newest EOL release (13.10) be shifted here?
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Okay, so I was able to get an answer on this from some of the archive/release team members. (The nitty-gritty gory procedures they use are documented here, though, but this is the basic information, here on this answer.)
I also have the log(s) of me asking this - see this page containing the logged chat on #ubuntu-release where I asked this.
According to the individual on the team(s) I talked to (cjwatson), the process of moving EOL releases from the mirrors to old-releases is a manual process and they do it whenever they get to it. There is no set time for when they move the data from the archives to the old-releases page, but they do eventually get around to it. The page I linked to detailing the process does explicitly state that "old-releases is by its nature not very regularly maintained" (quoted from the Wiki), so they do not regularly move the information over regularly and there is no schedule for it.