@Spage, I think this exercise might be useful to know which #dev.wikimedia.org documentation for 3rd party developers we want to prioritize.

This is related to T107211, but is distinct; T107211 is focussed specifically focussed on zero results, whereas this is more general.

Now done at https://commons.wikimedia.org/wiki/File:Search_api_analysis.pdf and https://github.com/wikimedia-research/ApiAnalysis .

Do we have any idea about who is doing these API queries, for which products, for which purposes?

Short answer, not really; as the report makes clear this is intentionally a high-level summary. We will dig into the sources more if and when we begin reforming how we expect users to format incoming search requests.

If you are interested in generalised reports on this sort of issue I would recommend putting a Phabricator card in the Research team's backlog.