For anyone interested in using this, it's still possible on build 327.10 from April 29, 2021 and older.

You can use this script with DepotDownloader. Extract DepotDownloader somewhere and create a .bat file with this script inside and run it. You will have to change "yoursteamuser" and "yourpassword" to your respective Steam information as you have to officially own the game in order to download any and all previous builds.

https://github.com/SteamRE/DepotDownloader

Script to add into a .bat file
dotnet DepotDownloader.dll -app 346110 -depot 346111 -manifest 4967374641183654644 -max-downloads 999 -username yoursteamuser -password yourpassword

This will download and install build 327.10. You will have to run the game in administrator otherwise it will crash on startup.

So far, I've only been able to get The Island working. Any mods made/updated after the Genesis Part 2 release (June 3, 2021) may cause issues. Creature mods updated after June 3rd have a 100% chance to crash your game when you open your inventory but older mods seem to work okay for the most part.

I may open a server for this if demand is high enough.