That may very well be the official stance, only officially supporting SteamOS in much the same way games used to only officially support a single major distro, often Ubuntu. However, I don’t see it actually stopping you from playing on other Linux systems. The functionality is there at that point. It’s just a matter of making sure you have whatever it depends on from the Deck.
That shit drives me nuts. Wanna be trusted with my life savings, but they can’t be bothered to implement modern security features until they’re already being phased out. I don’t know what will replace modern 2FA schemes, but I guarantee banks will adopt the current ones about three years after the replacements become standard.
Also, they’re charging you a poor tax for not having enough money, whether that’s a minimum balance or just accidentally spending a nickel more than you had on hand.
I don’t think Valve would help implement something like that. They’ve shown a lot of initiative in trying to push Linux into the mainstream for gaming, and a move like that would be counterproductive to that goal.
Plus, it ignores the fact that the Deck isn’t trying to be the only piece of hardware in the space, just the first to prove it’s commercially viable, and they succeeded in that. Competitive devices are coming to market, and when gamers start buying them, it’s going to seem foolish to whitelist JUST the Deck.
Rockstar’s only semi-viable play of that nature is to attempt to require SteamOS as your Linux distro, but I see no way to do that so you always and only block other Linux distros.