Darksiders 3 has two patches to choose between; this guide focuses on how to downpatch to the very first patch released on December 3rd for the Old Patch section.
If you have any trouble with my "quick" guide, please confer with the guide posted below. It has all the information here along with some pictures. It also explains how to manually downpatch a game yourself.
First, you'll need to access the Steam Console. This guide explains how to do so: https://steamcommunity.com/sharedfiles/filedetails/?id=1086279994
After accessing the console, paste this into the bar: download_depot 606280 606281 6389232169547293661
The game will NOT show up as downloading in your downloads, but you'll know it's working as your download speeds should act as if something is downloading all the same.
Once it's done, you'll need to copy the game over your current steam version. To do so, find it in: [path to Steam folder]/Steam/steamapps/content
Open up the Depot_606281 folder, and copy the files inside over to your actual Darksiders 3 Steam folder. This will wipe over your current download, but will play and run as an older version. Any saves will not be compatible.
After that, the last step is to check whether manual updating is set as off. To do so, open your ACF file with Notepad or something similar. Location: (Steam install path)/Steam/steamapps
The manifest you'll want is: appmanifest_606280
After opening that, look for the line that says StateFlags and make sure the quotations after it contain the number 4.
With that, everything should be set! You'll know you're running the correct version if your game is running Version 175,755 on the bottom right.