Can you play Once Human on the Steam Deck, ROG Ally, or Lenovo Legion Go?
The PC spec requirements for Once Human are fairly low, so hardware isn't an issue. Nor, it seems, is the Steam Deck running Linux. The initial problem is that Once Human doesn't have native controller support, so for the time being, it's best to hold off trying. You could, however, hook up a keyboard and mouse, but that rules out handheld play.
Steam Deck and ROG Ally hardware is fine for Once Human
The full PC specs for Once Human are a fairly low barrier to entry. Its minimum requirements are a now ancient Intel CPU, similarly old GPU and 8GB of RAM. The Steam Deck, ROG Ally, and other gaming handhelds easily exceed this. Realistically, they fall closer to the recommended specs, which is great to see.
While Windows 11 is naturally listed as the supported operating system, there doesn't seem to be a problem with running it on Linux, the OS of choice for the Steam Deck. Earlier tests and demos of Once Human have earned it a silver rating on ProtonDB, which while not perfect, indicates a solid enough playing experience. Albeit with some tinkering required.
Once Human has no controller support at launch
The problem with trying Once Human on a handheld right now is that there's absolutely zero native support at launch for controllers in the game. It's a PC-only launch (there's not even a hint of a console release right now) and it requires that age-old PC gaming input method, the keyboard and mouse.
Obviously, since handhelds are PCs, you could hook up a keyboard and mouse and play that way, especially if you have a dock and an external display. If your handheld is your primary PC gaming device, this will at least get you going, but you won't be able to put your feet up on the sofa very easily.
Controller support is coming, and the Steam Deck in particular has a strong community that builds custom controller layouts for games all the time, and it does, of course, have touchpads.
But those of us who just want to fire it up and get going have nothing at launch. Controller support is planned, but as yet has no time frame. It's frankly, disappointing, that a mobile version has already been announced for September, but such a basic feature is lacking for the players jumping in day one on PC.
My advice would be to hold off for now if you intend to play as handhelds are intended. It's a shame, but it's ultimately better than having to endure a compromised experience.