Maintenance of the interwiki map requires being able to find all existing pages which use a given interwiki prefix. The `iwlinks' table is very convenient for this. However, it only includes pages that have been edited or null-edited since the iwlinks table was added, i.e. when MediaWiki 1.17 was installed. (Purging, by contrast, updates the page_touched date but does not appear to update the iwlinks table.)
See the following three database queries for confirmation of the above:
This means that a lot of old pages are potentially missed in searches of the iwlinks table. See prior discussion here:
At the time of the discussion, @MZMcBride suggested instead null-editing every page. However, that seems like a Herculean task compared to running a maintenance script. Nobody ever got around to implementing either of these solutions.