This question is obsolete, as explained.
After asking about why it was not encouraged to ask downvoters to give a reason, I received a number of firm answers that contain a lot of good reasons (and some less good but that's not the point).
Now, only a few days or even hours after I was told there is no way downvoters would be asked such a thing, I downvoted a question and a notification appeared on top of my page reading:
Please consider adding a comment if you think this post can be improved.
Please understand my surprise. Have the rules of downvoting just changed?