How to update latest.civicrm.org?
You are correct that latest.civicrm.org/versions.json is the canonical source for CiviCRM version information.

The file is quite new and the release-publishing script has not yet been updated to write to it (an issue has been filed for this), so at the moment it must be updated by hand.

I agree that the file should be under version control.

If anyone is available and interested in helping with either task, get in touch on #civicrm IRC channel, or by email to coleman at civicrm)