I've installed Windows 11 on an external USB SSD. Changed to boot order on the deck so it tries USB first, and so all I need to do is connect the SSD and reboot to have Windows.
However, saying that, I doubt I'm going to use Windows much on it. Most stuff just works on SteamOS.
I really, really like this thing so far. I've even got used to using sticks in FPS games but those aren't my main reason for having the Deck. I'm mainly in to indie titles, RPGs, and dual-stick shooters these days.
Just hoping someone comes out with a nicely fitted external battery for it.