Such good news.
I hope someone can answer this either theoretically or practically as I’m not as knowledgeable in this.
One of the things I love about the steam deck is the ability to just turn it off and back on a few days later and the game is exactly where I left off. If steamOS is on a PC or another handheld deck. Would it still be possible to still have this feature? I guess my question is whether this is a software or hardware feature.
It's software. I'm pretty sure my linux desktop can do this... It's not a special feature, exactly, the system state gets saved to RAM, and then the CPU goes to sleep.
On resume the kernel reads the state from RAM and puts everything back where it was and things continue from the exact same point from which they were suspended. Theoretically.
It's a complex sequence, and windows sleep is famous for getting it wrong on lots of hardware configs. I've had trouble with it on linux, as well, almost always relating to the GPU.
Valve very likely put in some work to have it work as well as it does on SteamDeck, but theres no reason it couldn't work on any given device.
I'm using HoloISO (it's like 95% SteamOS) on a mini PC (all AMD, 680M iGPU because I wanted to get close to the deck specs). I mostly stream games from elsewhere in the house, but it has a few titles installed locally.
The sleep works perfectly so far for local titles. I assume other Arch based distros with all of the steam software installed (like ChimeraOS) work just as well. If the hardware maker who puts it on their box makes sure their hardware is well supported it shouldn't be an issue.