Valve have no plans for a Game Pass-style subscription service, according to Gabe Newell. They're not against helping Microsoft launch Game Pass on Steam, though.
"I don't think it's something that we think we need to do ourselves, building a subscription service at this time," Newell said when talking to PC Gamer. "But for their customers it's clearly a popular option, and we'd be more than happy to work with them to get that on Steam."
There's precedent for third-party subscription services being offered through Steam, as EA Play, EA's game subscription, launched on Steam in 2020. Microsoft have also cosied up to Steam recently, launching games like Flight Simulator and Forza Horizon 5 on the platform. Earlier this week it was announced that the Bethesda launcher would shut and all Bethesda games would launch on Steam in future.
While Game Pass isn't available through Steam, you will be able to dual boot into Windows on the Steam Deck, meaning putting Game Pass on there yourself should be fairly straightforward.
We also spoke to Newell this week in the build up to the launch of the Steam Deck - which we've now reviewed. We asked him what he thought of the trend of consolidation in the games industry, which Microsoft are at the forefront of. He said that consolidation "must create value for customers." He also told us that people selling NFTs are "not people you want to be doing business with", and many other things in our full Gabe Newell interview.