I was surprised to learn, as the National Law Journal reported today:
The recent settlement of the long-running legal dispute over modernization of the U.S. Supreme Court building included a pledge by the government not to give the contractor any negative ratings.
Read more here (subscription required).
I imagine it's unusual to see an entity with the size and strength of the federal government agree to this kind of clause.
Unlike non-disparagement clauses that we here at Public Citizen have challenged in the past (see here and here, for instance), this type of provision in a settlement agreement with both parties represented by counsel is not likely to be held unconscionable.