First things first, you dont need to do that to get playable framerates on this game, it is listed as "unsupported" saying "the games settings cannot be adjusted to run well on steamdeck" but it will detect its running on steamdeck hardware and it will hide most settings and it will use a steamdeck profile which is below low settings.
This looks like a last minute thing they put in because it looks like trash, at least with the provided upscalers (TSR /XESS), with those options it will look extremly blurry.
I would consider it deserves the tag "playable" at least until they implement FSR 3 nativley.

EDIT: I'am aware of some guides on youtube that use the SteamDeck=0 %command% launch option which will esentially do what happens after you patched in FSR 3.
I personally would not recommend that since to run the game on even the low settings you absolutley have to utilize upscaling at least 50% renderscale which would be performance on DLSS to FSR mod, XESS on Balanced or TSR on low.
Yes the SteamDeck preset is lower than low settings thats true but lower quality shading etc. still look better on ~75-100%, than 50% renderscale, which just looks like pixelated mush the moment you start moving especially on 800p.

To inject DLSS to FSR mod we will use Decky FrameGen[github.com] plugin, its the simplest, easiest and most universal way to do it, after its installed you simply select a game from the gamemode menu and hit on patch.

If you haven't done so allready install Decky Loader[decky.xyz], i wont provide further information on how to set it up its pretty simple and there are hundreads guides on how to do that allready.

When its installed browse the plugin store and install the Decky-Framegen plugin, if its not there you might need to switch the store channel to the "testing" channel.
After this is done navigate to the plugin in the menu and hit on "install FG mod", then select any game with dlss support you want to patch in this case Expedition 33, hit on patch and boot up the game.

Now you should be able to select DLSS in the graphics settings which will be FSR 3 in disguise you wont be able to enable any frame generation with this one since the game did not implement it but you will be able to use the upscaler, additionally all the other settings which will probbably default to "Low" are visible now too.

This is a problem since the deck definitley is not able to run the game on playable framerates with those settings, therefore we need to regain the steamdeck preset to run that game at resonable framerates.

To do that exit the game and navigate to the games launch options (gear icon->Properties->General->Launch Options} and remove the entry Decky-Framegen inserted there, dont worry the DLSS setting will remain adjustible.

Reboot the game and set the DLSS quality to your liking, i use DLAA with a 30 FPS limit.
It runs like a charm, it dips from time to time in crowded areas like in the prologue to around 27fps but its really not to noticable and for most part it will hit that stable 30.
You cannot disable V-Sync on the steam deck preset for some reason.
Other effects like motion blur, Film Grain, Chromatic Abberation and Vignette shouldn't have much effect on performance just enable or disable to your prefference, disabling all 4 will have most visual clarity but you could argue the games artistic style is build around those effects, actually one of the few games where i actually prefer that look.

Sure you wont get 60fps, the deck is still a handheld afterall.
Sacrifices have to be made you can not expect to do 60 on a brand new game like this.
But with this its solid, i'm actally blown away, how good it is with those adjustments.

EDIT: As mentioned by Abeltensor FSR Quality provides a much more consistend 30fps, on DLAA the game struggles on the world map a bit 27-30fps. I personally think its bearable and Quality level upscaling itroduces more fuss especially on grass and hair, but if you prefere fps over visuals you'll get a much more stable experience this way and it looks okeyish, i would recommend FSR on high quality which is something in between, at least on my deck i didnt see much of a performance difference.
Only on FSR balanced or performance i start to get a real fps boost again35-40fps but i wouldn't reccomend that, to land the QTEs its much better to have consistent 30fps with consistent frametime so your input lag is the same every single time.
Just take the extra battery and squeeze a few more minutes out of the deck if you can bear balanced/performance level upscaling.

There is a UE5 Tweaks Mod[www.nexusmods.com] for the game specifically designed for lower end hardware without potatoing the game like the original steamdeck preset does in some areas. Didn't have the time to really test it out lately, but i think its worth trying out, download the "Optimized Tweaks BOOST" Main File and follow the install description.
Hopefully you should be fine unlocking the games settings and still maintaining playable fps with an acceptable FSR level.
Either do not delete the launchoptions the Decky FrameGen plugin inserted or use "SteamDeck=0 %command%" to do this.
I'll report here as soon as i find the time to playtest the mod, especially in those areas that either looked terrible with steamdeck preset or gave me trouble with unlocked game settings.

There you go.
The image quality in terms of visual clarity should be majorly improved with almost same framerates.
Really don't know why they didn't implement FSR 3 natively maybe we get it in a future patch which would justify a "verrified" rating in my opinion, at least up until now.
I'm just 3 hours into in the game but i doubt it will get more demanding than the prologue, crowded areas tend to be the most demanding and due to the games story i doubt it will get as crowded as in the prologue anytime soon, just wanted to share what did bother me and how i improved my experience significantly.

PS: I dont think its neccesary but i did use CryoUtilities to optimize my deck in the past.