If a user experiences a sudden drop in karma, and a lot of −1 posts, they should be able to report the user, and a mod should be able to check and punish them and fix the problem.
I’ve never seen a mod capable of checking who downvoted what reacting in any way when thishascomeup.
I’ve never seen a mod capable of checking who downvoted what reacting in any way when this has come up.
I would guess that a mod being capable of checking that would be an easier or at least not much harder fix than a time limit on voting down.