However, alongside the site cleanup efforts, it appears that the impending approach of finals, standardized exams, etc. is resulting in an uptick in lower quality, "HOW I DOZ THE PORBLM" sorts of questions on the site. This got me curious as to whether Chem.SE has seen similar inrushes of lower-quality questions around this time every year.
So, naturally, I wrote a query. Disappointingly(?), no such trend is evident.
However, out of curiosity I ran the query on some other sites. The results on SuperUser were striking (click to enlarge):
The yellow trace is the total number of questions posted to the site per month, including questions that have since been deleted. The blue curve includes only questions that remain non-deleted; the red curve is the difference between these two. The green counts questions posted that are currently closed, but not deleted.
What did the Stack Exchange network do in May 2016 that reduced the number of low-quality questions so dramatically? I looked around Meta.SE a bit to see if I could find anything relevant, but without success. This Meta.SE answer talking about the hoops new askers have to go through is from 2014, and so is an unlikely explanation for the change. The second iteration of the SEQP was under heavy discussion in Oct 2016, well after this drop occurred, there doesn't appear to be anything around that time in se-quality-project, and a search of Meta.SE over the first half of 2016 yields nothing particularly illuminating.
FWIW, a similar drop is evident for the other major SO properties (click thumbnails to enlarge):
However, the drop is absent for most of the other sub-sites I checked—viz., Software Engineering, AskDifferent, Arqade, Physics, Math and Chemistry. Unix & Linux did show a slight drop around that same time, but it looks like more of a correction to an aberrantly high number of bad questions that the site received in April 2016.
So, again: What did Stack Exchange do in May 2016 that has had such success in discouraging poor-quality questions on the major sub-sites?