01The one Deck wrinkle

SteamOS uses an immutable filesystem: the system root is mounted read-only, so the usual "just install .NET with the package manager" doesn't work cleanly — and the fixes (unlocking the FS, flatpak, distrobox) are more hassle than this job deserves. Your home directory is read/write, though, and that's all DepotDownloader needs. So the single decision that makes Deck easy is: use the self-contained release. It bundles the .NET runtime inside the binary, so there's nothing to install system-wide — you download it to your home folder and run it there. Skip the framework-dependent build and the whole "install .NET on SteamOS" rabbit hole disappears. (This is the same self-contained-vs-framework choice the installation page makes for every Linux box; on the Deck it's not just easier, it's the difference between a five-minute job and an afternoon.)

02Desktop Mode & Konsole

DepotDownloader is a command-line tool, so you'll want a terminal, which means Desktop Mode:

Hold the power button → Switch to Desktop. (A keyboard helps a lot here; the on-screen one is STEAM + X.)
Open Konsole from the taskbar or app launcher — that's your terminal.
Everything below happens in Konsole, in your home directory (cd ~).

03Get it and run it

From Konsole, grab the Linux self-contained build, make it executable, and run it — the same command you'd use anywhere, just on the Deck:

# in your home folder
$ chmod +x ./DepotDownloader
$ ./DepotDownloader -app <appid> -depot <depotid> -manifest <manifestid> \
    -username you -remember-password -dir ~/old-build

The chmod +x matters: zip archives don't carry the execute bit, so the binary lands inert until you set it — the most common "nothing happens" on any Linux unzip, the Deck included. That's the one that bit me on my first run — the binary was sitting there inert until I set the execute bit. Get your three IDs from SteamDB, and if you want to log in without typing your password on a handheld, DepotDownloader's QR login is genuinely nice on the Deck — scan it with the Steam mobile app. Download somewhere on your home partition with room (~/old-build, or a folder on your microSD).

04Where games live on the Deck

To use the downloaded build you need to know where the Deck keeps games — and it's not the Windows path the guides assume:

Internal storage: ~/.local/share/Steam/steamapps/common/<Game>/ (the path ~/.steam/steam is a symlink to ~/.local/share/Steam, so either works).
microSD card: under the card's mount, typically /run/media/deck/<card>/steamapps/common/<Game>/ — check which drive the game is installed on in Steam, since the Deck spreads games across both.

From there it's the same three routes as anywhere — run the build standalone, copy it over the install, or have Steam adopt it — all walked through in after the download. (Konsole tip: enable "Show Hidden Files" in Dolphin, or just cd directly, since .local and .steam are hidden folders.)

05Pin it so it stays

Back in Gaming Mode (or in Desktop Mode's Steam), stop the Deck from quietly updating your downgraded game: the game's Properties → Updates → "Only update this game when I launch it", and for extra hold, the read-only appmanifest_<appid>.acf trick from the downgrade guide works on SteamOS exactly as on a PC (the .acf lives in your steamapps folder). Keeping the pinned build in its own folder and launching it as a non-Steam game is the surest "Steam never touches it" route, same as on desktop — on my own Deck that's the route I'd pick, since Gaming Mode is eager to update things the moment you forget. Cloud saves: turn them off for the game while you're on the old version, or sync will fight you.

06Prefer not to use a terminal?

The Deck is a handheld, and Konsole on a 7-inch screen isn't everyone's idea of fun. There's a graphical wrapper — SteamDepotDownloaderGUI — that's cross-platform and runs on the Deck in Desktop Mode, turning the same job into a form you fill in. It's the same engine underneath, so everything on this page still applies (self-contained, home directory, the steamapps paths); you're just clicking instead of typing. Either way, once the files are down, the Deck is just a small Linux PC, and the rest of this site applies unchanged.