As of July 2023, it's still possible to download UberStrike from Steam, although it's not listed on the Store anymore.
To download UberStrike, you can use either Steam or DepotDownloader.

ℹ️ Paradise only supports the current Steam build of UberStrike (v4.7.1). Basic support for legacy versions (such as v4.3.10.1) is included, but incomplete and unsupported (see Legacy Support).

ℹ️ In case UberStrike's files are removed for good on the Steam servers, we have put together the UberStrike Archive containing the latest Steam builds for Windows and macOS, as well as Windows installers and loose files for legacy versions (v4.3.8, v4.3.9 and v4.3.10.1).

⚠️ UberStrike is a 32-bit only application and WILL NOT run on macOS Catalina (10.15) or later.

• Open a browser window or the Windows "Run" dialog (Windows + R)
  • Using a browser window is supported on both Windows and macOS
• Enter steam://install/291210 and hit Enter
• Confirm the installation location in Steam and wait until the download is finished

• Open the console
  • The console can be activated by opening steam://open/console in a browser window or Windows "Run" dialog, or by launching Steam with the -console parameter
• Copy and paste the following command and hit Return to execute it: download_depot 291210 291212 8147831209968313515
  • For macOS, use download_depot 291210 291211 7198107249301931430
  • If there is an error that you do not own this game (or something along those lines), type app_license_request 291210 to add UberStrike to your owned games
  • Up-to-date depot manifest IDs can be found on SteamDB: Windows, macOS
• Wait for the console to print Depot download complete. There is no visible download progress.
• The downloaded files are located in <path to Steam>/steamapps/common/content/app_291210/app_29121x

• Open a console window and change to the directory containing DepotDownloader.exe
• Run the following command:
  • Windows: .\DepotDownloader.exe -app 291210 -depot 291212 -username <user>
  • macOS: .\DepotDownloader.exe -app 291210 -depot 291211 -username <user>
  • Before the download starts, you will be asked for your password and (if set up) a Steam Guard code
• The downloaded files are located in <path to DepotDownloader>\depots\29221x\696170

... because you have been playing on a different server, you will need to clean your game files first.
If the game is actually installed (not just downloaded) via Steam, right-click on UberStrike in your Steam Library, select Properties → Local Files → Verify integrity of game files. Steam will then download clean versions of any files that may have been modified. If you like to do things manually, you can always download a clean copy of the game files as described above and just replace all files in the game directory.

Getting clean game files is always a first troubleshooting step in case something appears broken on your end. Most of the times it's just easier to reinstall everything than trying to fix broken things. Cleaning the game files will not reset your progress on any other server (at least not on Paradise), and once you'd like to play on any server other than Paradise, clean your game files again, reinstall their patches and you should be good to go.