Gaming

I don't know what you mean by "steamdeck version", but afaik the SteamOS you can download from steam is version 2 which is really old by now while the Steamdeck runs version 3 and is still not available for download outside of it. Mind you it isn't just a major version change but they even changed the linux OS it's based on, moving from Debian to Arch.
As for the replacement for desktop, the Steamdeck (and with it SteamOS 3) launched with full desktop mode available and while the core system is immutable as discussed, it's still a fully functional desktop OS and as such it depends on your personal needs how much of a windows replacement it can be, just like any other linux OS. Also you might need a dock or other USB hub to attach a keyboard and mouse if you need them, but there is onscreen keyboard and touchpad on the steamdeck available.

I don't use it as a desktop replacement (I still have a windows desktop). But I do but default have my deck docked and hooked up to a TV, in desktop mode.

I've installed various flatpack programs, to include discord, chrome, and a couple jetbrains ides for various languages. They work great. I've done a couple non steam games (diablo), with mild annoyance but success in the end.

I have not used any office programs on it (as much as it's cool to like Linux, I just think Microsoft wins by a lot there, and have no desire to get them working on Linux).

The deck itself has some power limitations, but I have no reason to think that they would translate to steam os on a more powerful machine.

So basically, it probably depends on what you want. It's pretty reasonable. Definitely good enough for my secondary computer that blocks ads for videos on my tv. But personally, I still want a windows computer, because windows is the default assumption for most software I use.