Is the statement "If you talk to so-and-so woman, you are unlawful to me" – which was said as a threat and not intended as a divorce or a ẓihār – considered a ẓihār or a divorce, or does it necessitate an expiation for an oath, especially after the other woman contacted the wife without knowledge of the threat?

If the husband says to his wife: "You are unlawful to me" (anti haram 'alayya), and he does not intend divorce or Dhihar, then the preponderant ruling according to scholars is that this falls under the category of an oath. No divorce or Dhihar occurs by it. The husband must offer expiation for an oath (kaffarat yamin) if his wife speaks to that woman.