I've gone over Story Ideas myself. Multiple topics since the site's beginning remain standing, even if inactive. Thus, any purging would have been done manually.
I then dug deeper using my privileges as a moderator. Of the topics from Story Ideas that have been deleted, only one has been since your joining. It had its content purged by the creator back in 2014 once he requested the topic deleted; due to not hitting the report button or PMing someone, that request fell through the cracks. In the interest of completeness, I've gone over every forum. Discounting troll attempts and advertisements, there was only one such topic. However, it was not created until last year, which your window eliminates as a candidate.
Therefore, unless the topic was given a hard delete - meaning there's no record the topic was ever there and as such cannot be restored or viewed - or your window is wrong, there was no such topic.
