Unless it’s either PS5 or Xbox exclusive (not both), I don’t think that’s true. Sony and Microsoft wouldn’t collude to prevent launch on PC. That’s extremely illegal, even for companies that are masters of dodging antitrust laws.

The most realistic explanation (IMO) is that Rockstar did their research and found that most PC players also own a console, and will very likely buy the game twice in the long run.

Or if we’re being charitable, maybe the game needs more optimization work before it can run well on the Steam deck, and they want that working before launching on PC.