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They have added an undown link a while back.

• yoz-y 7y ago
  But the undown/unvote links are hard to tap on mobile as well. On firefox on android it's always an exercise in patience.
• slavik81 7y ago
  Adding undo changed the feel of downvotes a little. Irreversible accidental downvotes were scattered randomly on comments and they made the occasional downvote feel less harsh.