Of course they don’t, it would be crazy to say they would support all the different possible distros and configurations that people might run, when the majority of users are in steam deck. But that doesn’t mean it won’t run, just that if you have issues, they don’t promise to fix them. Seems reasonable to me.

BG3 already ran well enough on Linux, so I imagine this will only make it run better, official support or not.

I'd expect it to work anyway. Under Steam at least. There is nothing special about the Steam Deck/SteamOS that's not available on other distros when running Steam, afaik.

Well, most game companies will only tell you the game only works on something very specific, say Ubuntu 24.04, and everything else is untested/unsupported. That doesn't exclude the game will work perfectly fine on other distros, which is usually the case.