Firstly, I would like to know why this question was deleted. I feel the question was a good one, and I'm also sad to see my answer to it disappear. I'm sure there was a good reason for deleting it, but I have no idea what that reason is.
This leads into my more general question, which is, what should a user of mathSE do if a question or answer they're following is deleted in this way? I don't think that "nothing" is a reasonable answer here; users want to know why the question was deleted and if it will ever come back, and I would say they deserve to know.
I think the most common reason a question could be deleted "for reasons of moderation" would be that it is part of an ongoing contest or test. But in this case, it should eventually return. Is there any case where a question is deleted "for reasons of moderation" but never returns?