
PROTON_USE_D9VK=1 %command% -console -nointro
Multiplayer via the alterware client is the best way to play this. Since it's safer.
For multiplayer use the alterware client, run the alterware file with the argument --update to install the files for iw4x and then add the iw4x exe file to steam as a non steam game and use the launch options

No tinkering. Just press play and enjoy :)

It is not reccomended to use Steam matchmaking. Instead use the IW4x client provided by AlterWare.
I didn't try to launch the vanilla version of this game as matchmaking is very dead on Steam servers. IW4x was very easy to set up however and brings new life into this game. Highly reccomended.

Custom TDP limit set - see other notes for details
Steam Input simulated mouse can misbehave depending on graphical settings - see other notes
There seems to be a consistent timing problem between the frame drawing/sync and the Steam Input control handling. If you play in a "default" state (Deck's FPS limiter on) and simulated mouse input like the Joystick Mouse, the movement is extremely stuttery and jumpy despite the game running at a reliable FPS. In-game v-sync ("sync every frame", off by default) also causes this bug.
Disabling the Deck FPS limiter solves this problem completely. However, the game then runs at several hundred FPS, which is a complete waste of battery power. Setting a custom TDP limit solves that. I've found 7 watts gives a few hours battery life without any noticable drops below the OLED's 90 Hz. If you don't need 90+ FPS, you can lower it a couple more watts and it still plays great.
Besides that, this is standard fare for a PC game of its vintage - set the screen res correctly, figure out the keyboard controller mapping, etc. No real tricky tweaks, just the right settings and it's excellently playable.

gamescope -f -W 3840 -H 2160 --force-grab-cursor -- %command%
Multiplayer: crushed quite often and experienced lag when starting a multiplayer game until I've set iommu=pt in grub settings - after that no crashes or lag occured.
set Vsync on and to 60hz in order to get smooth frames - leaving vsync off is not enjoyable.

Looking around and aiming with the right stick is very inaccurate and jumpy. For example, if you hold the right stick fully to the left, expecting the character to spin leftward, the speed at which the character turns is erratic (slowing and then going faster, randomly) and makes aiming accurately impossible. I wasn't able to fix this with my controller layouts and in-game settings tweaks.
Would crash occasionally when changing video settings. Had to launch in safe mode a couple times.
Wasn't able to find a game. Kept connecting with only one player, who was a modder advertising a Discord server, he would leave without playing.
Unplayable on SD solely because of the issues when looking around with the right stick.
the singleplayer wont start

nothing wrong with it just a couple of quirks here and there
could not enable windowed mode most often, after around 5 mins of playing around it randomly switched to windowed mode using Meta+Enter
mouse would escape the game window sometimes after tabbing back in

gamemoderun gamescope -W 3440 -H 1440 -f -e -r 165 -- %command%
Tweaks made for precise mouse sensitivty control, FOV adjustments (didn't work), setting as read-only causes the game to freeze if on ext4 and is not possible on NTFS so there is only so much config editing you can do.
The older Call of Duty games are not compatible with some XOrg multi-monitor setups, depending on the DE you use. Older DEs and WMs which don't correct mouse snapping bugs (your cursor snapping to the center of the virtual X display across all your montiors) will cause the game to be unplayable since the camera can't be controlled. This can be fixed with Gamescope, but that causes other performance issues.
Framerate felt inconsistent, even when aiming for a low framerate target.
Freezes on occasion especially when saving/loading config, force quit breaks Proton permanently until system restart.
Networking was an absolute nightmare, more packet loss than you can shake a stick at and it got worse and worse the higher the framerate was.
Only works for me on Steam Deck w/ controller input, significant game-breaking issues on desktop. Requires Gamescope to function.
May be host related but connection wise made me feel like I was playing on Xbox 360 where an enemy running accross the map was frame skipping.
Very rare lag spikes

Its an easy setup so long as your okay with changing the default resolution and finding a custom controller configuration.
Trigger behavior was set-up weird. Changed it to have hold to aim down sights
Game runs 90fps on my oled Deck with no noticable drops. Great if you just want to kick back and play the campaign for some nostalgia. I did not try to play online with there public servers nor did I try to play on a local server with other deckers/PC's so i have nothing to report on that content. Game deserves to be platinum the campaign and spec-ops. Overall - 10/10

The game completely freezes after 5 games in multiplayer on any version of steam proton. On one version of steam, proton freezes completely when entering the game. Usually the game freezes as soon as the map starts loading and happens consistently after 5-7 games.
Game freezes completely after 5 games in multiplayer

Perfect OOTB

PROTON_USE_WINED3D11=1 gamemoderun %command%
you have to change the iw4sp.exe with a cracked one to make it work. I found one here : https://is.gd/gXWt1c

After using Steam Overlay, the mouse would be stuck off-center, so moving the mouse around in game would result is extreme sensitivity.
(Campaign) The game is unable to change level and will get stuck on a black screen, having to force-close the game, and manually selecting the new level from the menu, since "Resume game" will also hang on a black screen.
Game works flawlessly otherwise.

I used
I had to disable my IGPU in the BIOS to use the GPU
I used steam tinker launch to change the fov with CoD.SCZ.FoV.Changer by first choosing "Main Menu" then on the bottom row "Game Menu" then under the "Misc Options" section enable "Use Custom Command" and "Fork Custom Command" then change custom command to CoD.SCZ.FoV.Changer this should now allow the program to see the game.
gamemoderun %command%
Works flawlessly.
Some issues tinkering with controls otherwise all good
Turned off Gyro because I dont like it, ingame control settings set melee to v and ADS to hold. Adjusted mouse sensetivity to preference.
60fps with no issues on current release of proton on SD. Just wish it had native controller support like the console ports.
Game instantly crashes unfortunately. Also tried proton experimental, and proton 3.7-8 with the same results.
Does not boot
No problem, everything runs smooth using Proton 8.0 i did not test the multiplayer, only the singleplayer.

Refresh Rate capped @ 40Hz and FPS locked to 40FPS.
Somewhat hard to read text, but it is legible enough if you are not awfully far away.
Occasional crackling, not an issue.
Intermittent stutters. Not game-breaking.
Play it, you'll enjoy it.
No problemo

gamescope -w 1920 -h 1080 -r 165 -- gamemoderun DXVK_ASYNC=1 %command%
Using a kernel with the PDS scheduler alongside setting the DXVK_ASYNC=1
seem to be required for these older CoD titles to work well. With the settings I used there was no suttering, hitching, or fps drops. Everything seemed to function as good or better than it does on Windows (eg. the alt+tab bug present on windows doesn't seem present when using gamescope).
Game doesn't start.
DXVK_AYSNC=1
i always use DXVK_ASYNC=1 with GE proton to prevent stuttering and use FSR for FPS games. i also used Steam Tinker Launch to set up a custom inject command to launch CoD SCZ FoV Changer (https://github.com/marvinlehmann/CoD-SCZ-FoV-Changer/releases) to change the FOV
In multiplayer, there was no audio at all
Just download one of the community control layouts and you are set for Single/Spec Ops. (In my opinion still janky, since there is no offical controller support). Spec Ops multiplayer doesnt work, just keeps connecting to IW servers.
The multiplayer part works fine, but I had no audio at all. Had stable 60FPS in both single and multiplayer.
40 frame limit/40 refresh rate. TDP 5 watts/ GPU 600. No FSR needed.
First window: Only if you use the Deck to exit the game. If you quit/exit the game within the game, you should not get this window. This is about the game not quitting properly. Go in safe mode? Tap on No. Second window: about your computer Changed. = Tap on No. Game will then resume to the menus. Again. NO on both windows/boxes.
This report is for the campaign portion, and spec ops. I did not DL the multi portion. That is a separate 4+gb.
Works great OOTB
Just install and hit play, 5h in with no issues (offline mode), mp tested & works, SP special ops works as well
R1 to G L1 to 1 B to C key R3 to Ctrl L4 to Q (lean) R4 to E (lean) Left trackpad changed to numbers needed for weapons
Processor - 6W GPU clock - 600Mhz Performance: 50-60fps - around 10W TDP draw
Besides hackers, obviously.
Split Screen works fine
Decent.
Works just fine.
There were some momentarily freezes but they were rare.
Didn't try multiplayer.

Spec ops is not working. It cant connect.

Just works
Applying changes when changing your refresh rate in settings menu causes a crash. It applied fine the next time.
Beware of hackers online. It's probably a dangerous online environment in general. And joining some lobbies from official matchmaking will bump you up to Lvl70 without your consent. But I can't deny that it's still a super fun game.
Yes, it's all good.
Multiplayer seemed to work without issues
Multiplayer seemed to work without issues
Did not test singleplayer
Game doesn't launch
Tried with default Proton, experimental and 6.3-8 but the game doesn't start at all