Why has the number of questions dropped sharply between September 2019 and November 2019 (and never recovered), despite the traffic staying constant?
Good to see you Franck. I'm not certain, but I think it was a result of two factors:
Software Recommendations was flooded with off-topic questions for a while due to questions being directed to our community via a somewhat ambiguous form on other Stack Exchange sites. I think Stack Exchange removed or modified that form around the time our new question volume returned to a more typical level.
I believe the sudden decline was around the time that Stack Exchange treated a moderator very unkindly, and consequently there was a mass exodus of active users and moderators from the Stack Exchange network.