I think this is a bad idea, for two main reasons.
The privacy of the anonymous voting can be compromised. Even if an user is effectively "dead and buried" in this forum, I would still prefer to maintain their privacy in their voting preferences. A little bit of detective work is all that's required to connect the dots between date of deletion of the account and the date of the vote removals.
Why exactly is this information useful to you? For users' active downvotes and upvotes, knowing which posts were voted up or down can help the poster tune his/her writing to be better in the future, and also indicate to the poster possible mistakes made in past postings. But there really is nothing you, as a user, can do (in terms of amending your posts) to prevent or counteract the effect of a user-account deletion.
The key is your framing of your question: you said that a post "suffered" from this drop. I firstly disagree with that choice of word, and even supposing I do agree, considering that there's nothing you can do to ameliorate said suffering, what's the point of knowing this information?
Given that I don't see any benefit for having this information handy, and I can see a possible minus in terms of privacy (albeit rather small and unlikely) should this suggestion be implemented, I must disagree with the feature request (at least until I am convinced that the benefits outweighs the flaws).