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.

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.

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.

Or just Proton

Yes, but native is always native.

Something I wonder is if this new version is a linux build specifically targeting the deck hardware+OS setup, have Larian now committed themselves to following whatever Valve does in future for changes to that setup. In any case, they've got a fallback which is the windows version on proton, but it's inverting how Valve has trained many to behave which is to make just a windows version and delegate linux support to them.

There's also been persistent speculation about whether Valve would take on the burden of releasing SteamOS as a general distribution anyone can install on their own hardware (which I think is unlikely), which could in turn affect how Larian has to treat this port even if that is just communicating what it is and isn't.