It is still a nominal requirement that all US travelers purchase Cuban health insurance for the duration of their trip. We ask about it each time we travel and the doctors at baggage claim typically respond with 'Did the immigration official ask you for it when they stamped your visa?' in answer to our questions.
The truth is that the airlines have also contracted with ASISTUR (Cuban agency that provides this insurance) to cover you during your travels but they are shy about stating such likely due to the amorphous language from the US Department of Treasury's Office of Foreign Assets Control (OFAC). But, with a receipt of purchase for your plane tickets and boarding passes (keep physical copies of these) you should have access to the Cuban medical system should you need it.
We carry an additional form from the ASISTUR website that we present to medical providers when the need arises just to make sure there are no surprises. We also counsel our clients to purchase emergency repatriation insurance to cover the cost of that eventuality.