I didn't use the page category to assess the current number of stories because I attributed the difference to a delay in updating. However, if the update is not immediate, the difference was due mostly to another reason: the categories page has also attached to the fandom crossovers (seriously, you knew that?).
The number of missing fics, was therefore added that of the fandoms's crossovers. This explains the result for Buffy and Dr Who.
In short, the previous picture is completely false. (mea culpa, mea culpa)
In case your confidence in my statistics has not completely disappeared...
It turns out that I had made a recovery of categories' pages on the May 30. When the result is compared to that of pages retrieved May 28 (published on wikipedia), you get this:
So, good news, few stories have actually disappeared (which does not alter the fact that it is unpleasant for authors and readers concerned).