We'll see. It's a true current gen game and the Steam Deck is very much portable PS4 territory. The requirements currently list 125GB SSD storage as a requirement so I'm curious if it's even doable on the 64GB Steam Deck or how well it runs off a microSD card. Maybe it will work OK at low since it's aiming for 720p but it might be worth holding off especially if you only want to play Starfield.
I mean The Last of Us Part 1 is a next gen game and that's Deck verified. Same with Spider-Man Remastered, Miles Morales, and Ratchet and Clank Rift Apart. Plus Cyberpunk is Deck verified and that game struggles on even midrange systems.
With the minimum specs for Starfield I don't see how there could be any issues running it on Deck when all of Bethesda's other games, including Ghostwire Tokyo and Deathloop (both being next gen exclusive games) are Deck verified.