Some people did. Some people left for other reasons. One of those reasons was disliking getting downvoted a lot. In one case, it was specifically disliking getting mass-downvoted by Eugine. Which happens to be what you asked for.
(I agree that most people who have left LW have left for reasons other than getting mass-downvoted by Eugine. I hadn't thought that was under any sort of dispute.)
"Mod-bombed" is strange expression. I find it probable that at least some people left LW because of karma harassment. However my impression stands -- what made LW barren is people leaving because it stopped being interesting. But judging by the volume of discussion about particular reasons for leaving, you'd never guess that :-/
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.