almost 2 years ago - Munin - Direct link
It worked like this in the past, but remember: This way is not officially supported, trying at your own risk! The only official way to download older versions of a game is by the developers adding older versions to the Beta tab in the game's property settings in your library. For example, SCS Software does it, Iron Gate does not.

This is the main guide you want to use:
https://steamcommunity.com/sharedfiles/filedetails/?id=1086279994
But instead of downloading the Manifest with the help of Steam's console, use Depot Downloader instead. Steam client no longer allows you to download a Manifest you want, it always downloads the latest Manifest. So this is your only option.

Step 9 is important otherwise Steam will update the game.

Here's a guide which explains the use of Depot Downloader (primarily Steps 6 & 7):
https://steamcommunity.com/sharedfiles/filedetails/?id=2353930763
Note that, Depot Downloader requires credentials of your Steam account in order to make connection with the Steam servers and download the Manifests of the game.

Do so at your own risk.