Even if Starfleet classified it, don't you think the Klingons would have been publicly screaming every chance they got (Battle of Organia, Khitomer talks, etc) about that time Starfleet dishonorably tried to blow them up? I don't think it's something the Klingons would let the Federation forget...
Maybe this issue was part of the Khitomer Treaty. Perhaps Section 31 or Starfleet and the Klingon Intelligence Network arranged for a mutual classification of this issue for some reasons.
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