Steam allows you to download historical versions/patches of games through console commands as long as your account owns the game.

Why Does This Matter? Striking Distance Studios have unfortunately patched out some glitches in later versions of the game, like Gun Skip in Aftermath. Making it faster to play on older versions of the game to save time from being able to do certain glitches

How Do I Get Older Versions? Step 1: Open your web browser of choice and enter steam://open/console into the web address bar. Step 2: It will ask if it can open Steam, select yes. Step 3: Type the following into the console in Steam and press enter and it will begin downloading: download_depot 1544020 1544021 4540974869511074938

For full clarity, 1544020 is The Callisto Protocol game id, 1544021 is The Callisto Protocol depot id, and 4540974869511074938 is the manifest id for the patch. This is patch before the Hardcore mode was added. This is believed to be the optimal version of the game to play.

It's not perfectly clear how far along the download is at any point, but the console will tell you when the download is done, from there you will find it in steam/steamapps/content, under the same ids as before. Once its downloaded, copy the files from inside the folder into steam/steamapps/common/The Callisto Protocol

You're now good to go :)