Manual:DeleteArchivedRevisions.php

Details
deleteArchivedRevisions.php file is a maintenance script to delete all the archived (hidden from public view) revisions, by clearing out the table. These revisions will no longer be restorable. Deleted pages and archived revisions to a given page are permanently deleted, but the complete history of existing pages will remain.

When a page is 'deleted' by an administrator, records are moved from the 'revision' table to the 'archive' table, although the bulk of the data (page text) remains in the 'text' table. The script deletes all records from the archive table and then invokes to purge the text records which no revision links to. This can be useful for reducing the size of the database but it does mean you lose the ability to restore the according revisions.

Prior to MediaWiki 1.16, a helper file  contained the class that performs the work. This class was merged into deleteArchivedRevisions.php in 1.16.