This guide assumes that you

• have skills to use command line
  
• know when to trust your steam credentials to 3rd party tools

I intentionally did not make it super user friendly because i respect developers' decision to get rid of the old game build. So, you are not encouraged to play it, unless you absolutely have to.

First off, you don't need an old build if you don't have a savegame you're trying to finish. There are two possible locations, you should check them and back up all the files in case they get wiped by accident.
Location is %LOCALAPPDATA%\TSCGame - for example C:\Users\username\AppData\Local\TSCGame

Create a zip of this folder just to be sure you don't lose anything.

Steam has a nice page with all the files stored by each game in your account. The Sinking City files are here: https://psteamproxy.yuanyoumao.com/account/remotestorageapp/?appid=750130

Check out DATE WRITTEN column, they should correspond to your play time. Download all the files anyway.

TODO: i don't know if there is a way to restore files from steam cloud if they were deleted or overwritten. In my case, they were OK because i was careful enough to not launch the new version.

I did not investigate this properly, but old game version creates files named like ManualSave0.sav and new creates Frog_ManualSave0.sav - that's how you can tell which version your save files belong to.

Now to actually get files. Steam games are distributed in depots. Think of them roughly as archives or builds. Each time a game is updated, its depots get a new version. Versions here are called manifests. So, we are looking for depots of The Sinking City that were updated in January 2024 - when game was updated but old version was kept intact. And here it is: https://steamdb.info/depot/750133/manifests/

Depot 750133, Manifest 459041139380900076 from 16 January is the one we're looking for: it has a lot of files in a folder called OldGame.

Now it's time to use heavy artillery. There is a tool for the job: DepotDownloader[github.com].

IT WILL REQUIRE YOUR STEAM CREDENTIALS, USE AT YOUR OWN RISK!

Use it to download this specific depot version. If you have steam guard enabled, app will wait until you authorize it from your authenticator.

• Now wait for files to arrive
  
• Nuke everything in steam folder of The Sinking City
  
• Place downloaded files from OldGame folder there, so that TSCGame.exe is located at the top level
  
• Check that you have relevant savegame files in appdata
  
• You should be able to run game from steam and load your save!