It has been a couple weeks since i played, so I am not sure if patch 4 has anything to do with it. I have not found any posts about it not working on linux, so i was wondering if anyone else on linux or debian having trouble?
edit:
I switched to proton experimental and it works
Patch 4 broke Vulkan. Try launching on DirectX; it launches for me, but has horrible graphics quality. (Pixelated shading, shear edges of hair and plants.)
You don't need the launcher. You can run bg3_dx11.exe directly. You will need DXVK, of course, though that's already present if you're running it from Steam.
I've had some trouble getting my modded save to load at all, to load completely, to play dialogue correctly, and not to crash when fast traveling. Even after purging all my mods I've had problems. What I've learned: Don't use the lari launcher. Just use the exe from your game files, or the dx11 exe if that doesn't work (although for me skipping the launcher already solved my problem). If you're using a mod manager you can launch over those as well. Larian will probably hotfix over the next few days, but until then this is a nice workaround.