Prologue: This is an effort to follow up on this post which is marked as a duplicate of this highly voted and status-declined feature request (although they seem to be different, one focusing on the OP and the other on the downvoter).
My New Year's resolution was to downvote more. So on January 3rd or so, I saw a post that was low quality, and downvoted. Two days later, I came back to see if it was ready to be deleted or had gotten piled with downvotes, to find it with a significant positive score! At which point, I noticed that the answer had been significantly improved. So I retracted my downvote, changed it to an upvote, and immediately resumed my dearth of downvoting again.
I think I have a responsibility to downvote bad posts, despite that oh-so-painful -1 rep penalty. But if I am also going to have the responsibility to follow up on every downvote I cast, I'm not going to bother. I don't want my downvote to leave a residual penalty on posts that were once low quality, and I am not going to trouble myself to keep checking up on every post I downvoted.
Can a downvoter have an option to be notified when a post they voted down was edited? I wouldn't make this an automatic thing, because not all downvoters want it, and it isn't applicable to all cases of downvoting (although someone did suggest that this should be part of the responsibility of a downvoter).
Personally, I would make this a universal toggle on-off option on the user profile page, not a question-by-question option, but I don't have strong feelings about it.