s&box is the upcoming game from Facepunch Studios that acts as a sequel to Garry's Mod although it won't actually be Garry's Mod 2. Recently Facepunch were asked about Linux support since it uses Valve's Source 2 game engine.
Speaking on the GitHub request, as expected the idea was shot down by Garry Newman directly who said:
No plans to support Linux natively. While the deck is good to support - I've tested it and it works great. Could it run better if it was native and using vulkan? Probably. But it would also run better if you format it and install Windows on it too.
Any support for Linux outside of it running on the deck is incidental, it isn't supported officially.
So it's going to entirely rely on Steam Play Proton for the Steam Deck (like most other major games will) and it won't be supported in any way on Linux outside of being used on the Steam Deck.
It's not exactly a shock to know, considering Garry's previous comments about supporting Linux directly, along with the removal of the native version of Rust (which should work in Proton in future). The question though: is it a problem that they won't support it outside the Steam Deck? No. Well, not unless Valve support Proton differently than they do right now it shouldn't actually be a problem. Proton should work the same on the Steam Deck as it does for normal Linux distributions right now.