In my opinion, there are only 2 possible scenarios (if you doubt that he is insane/suicidal) for saying things out loud, that any other Russian citizen would long be in prison or dead for.
1. He isn't afraid of Putin, because he has some serious power behind him in form of intelligence or other higher up inside the Kremlin. In this case, Putin is afraid to order his execution because he risks an internal war for power and that would probably be the end of all his imperialistic ambitions.
2. It's all agreed with Putin, because they started to realize they can't achieve any goal of their invasion and they need to prepare for the worst possible outcome, meaning to admit their failure and leave. That means important heads need to roll. So Putin uses Prighozhin to slowly, but surely warm up the public to this and put all the blame on the MoD, namely Shoigu and Gerasimov. The narrative would be being lied to by his own MoD and therefore Putin ordered the "special operation" based on false information. Putin wouldn't look good, but he could still stay in power and their propagandists could publicly crucify the MoD.