Don't think he's officially retired, if Rizin wants to pay him stupid money to fight some old legend again maybe it will happen. But considering how he got bood in the Kosaka fight I wouldn't count ont it.
He was never seriously into it anyway. In interviews he called his MMA stint a hobby and amateur tier (though also admitted about daydreaming about the UFC). Like his first tweet after beating Fujita was cheering about not sustaining serious injuries. And as he's already rich, famous, proven himself in another sport and has a full time job being a celebrity in Japan he just lacks the hunger for it.