Packrat was an add-on for Dropbox Pro accounts. For $39 per year, a user could recover a previous version of a file, or a deleted file, no matter how old it is. This was a significant upgrade to Dropbox’s standard 30-day file version history.
The Packrat feature is no more, apparently. The cost of storing previous versions of files, forever, was too much for Dropbox.
Dropbox made the change sometime in August 2014, according to internet history at Archive.org (translation warning: the archived pages are in Chinese). The updated page replaces Packrat with Extended Version History, and now preserves up to one year of version history. The price remains the same.