It seems to me that there are not a lot of new questions on "elementary set theory"?
Have new tags been created? which would explain this apparent evaporation...
My understanding is that most of the time, someone who asks a question on an elementary topic will not be able to name precisely what the relevant tags are. In such cases, it's all up to more senior users to tidy things up. But then not all questions get re-tagged properly
Using Data Explorer (SEDE) you can check how has the number of questions in some given tag evolved over time. Here are two queries, one showing absolute number of questions in the elementary-set-theory, the other one showing percentage of all question which have this tag. The latter is taken from here: A statistical question about statistical questions.
One might be also interested in questions in a given tag which have lower quality. Of course, it depends on what you decide to take as an indicator of quality, but you can check, for example:
For comparison, here are similar queries for all questions (not restricted to a specific tag): average score since 2012, number of closed questions, percentage of closed questions, number of deleted questions, percentage of deleted questions
You can find some other SEDE queries related to tags here on meta. Apart from SEDE, you can find number of questions in some date range also by search - for example, all questions or closed questions in one month (in this tag).