For a week or so now, we've heard rumblings that Valve are preventing the creators of games with "mature themes" from releasing their games in Steam early access. It reads: "Your app has failed our review because we're unable to support the Early Access model of development for a game with mature themes. Please resubmit when your app is ready to launch without Early Access." The platform holder's early access submission documentation doesn't make any stipulations about "mature themes" at the time of writing, and there are mature-themed early access games currently in development, which suggests it's a recent shift of direction. I'll let you know as and when Valve update us on their early access policies.