Works perfectly fine
Borked, freezes as soon as you punch someone. Unfreezes after 1 min. Tried on default, GE, and experimental. Native doesn't seem to launch.
borked
I had a few issues with the native version of the game like my mouse not being picked up in-game and performance not being great even on the lowest settings. Switching over to proton experimental however got rid of the issues I had and even allowed me to play on max settings while getting a consistent 75fps, and my mouse even worked in-game. So if you do have any issues with the native version just try proton. I played with a Dualshock 4 and a Steam Controller and both worked without any issues.
Platinum. Works better than native.
Finished using Proton and Xbox 360 Controller, everything works. This game is Unity engine, so if there is problem with performance, try turning off shadows and water effects (it also works for native version).

No fix after it was reported by someone in July 2020: https://steamcommunity.com/app/260160/discussions/0/2566438792552775299/
Launched with this command: LD_LIBRARY_PATH="" %command% If you don't do this, game doesn't start unless you switch to the Steam Linux Runtime (and with that, there is no audio). With the command above, just use your native Linux libraries and everything works, including audio. Note that Proton 5.13-2 worked fine as well (including audio).
No audio with no launch flags, but using Steam Linux Runtime. Fixed with LD_LIBRARY_PATH="" flag and native Linux libraries.
With the LD_LIBRARY_PATH="" addition, it initially launched fullscreen with the aspect ratioj wrong on my 2560x1400 - I simply went into the game options and toggled Fullscreen off and then on again and the aspect ratio corrected itself.

The game works great, if you turn off shadows and water effects. Sometimes, when pausing the game, the cursor disappears, but this isn't a problem, just use the keyboard in the menu.

Game works native and on Proton out of the box. I installed manually D9VK and it worked as well fine with this DX9 to Vulkan translation (only some minor graphical glitches).