While much information should be public, a council is entitled to maintain confidentiality where it can be shown to be in the public interest.
In employment matters it is normal for some matters to be kept confidential and it is considered damagaing if they are not. In the particular case of the settlement of a claim to an employment tribunal, it is common practice to reach a compromise agreement between the parties, one part of which is pretty much always a confidentiality clause, binding on both parties.
Breaching that confidence is actionable. Although it would need someone to embark on legal action. It also looks to be a breach of standards, since it lays the council open to legal action.
This assumes that the decision to enter into the compromise agreement was properly taken by the council or by a team delegated to make the decision by the council.