Back in 2016, the US Department of Justice warned HR professionals that no-poach and wage-fixing agreements were illegal per se, and that violations could involve criminal charges.
A per se violation means that the existence of the agreement is illegal; there does not need to be an allegation of actual harm to an individual.
Earlier this month, criminal charges were filed against Surgical Care Affiliates and a related entity for agreeing with competitors not to solicit each other's senior-level employees. This is considered a form of collusion, a violation of the Sherman Anti-Trust Act.
You can read more here: https://www.hrdive.com/news/doj-brings-first-criminal-no-poach-charge-after-warning-hr-pros-of-prison-t/594049