NOTE: if you are trying to use Steam's Matchmaking system, this guide is actually not necessary. You can do so by simply adding a launch option.
Right click on the game in your Steam library, go to Properties, then down to Launch options add -steamMM
Obviously you will only be able to play with people who have this same option enabled, but you will be able to stay up to date with bug fixes and any added content. This still requires the EOS .dll file to be present.
Source: https://www.reddit.com/r/paydaytheheist/comments/147im97/theres_a_launch_command_to_restore_steam/?utm_medium=android_app&utm_source=share

Backup your saves before doing anything! (see next section), then disable Steam Cloud saves. You can do this by right clicking on the game, Properties, and unchecking Cloud Saves.

Once you decide to go back to the current version, understand that your progress could be corrupted and you may have to restore from the save you backed up. So hold on tight to it.

Not everything may work and the game could crash.

You won't see many people online because you can only join people running the same version.

You may have to do this every time there is an update. This is why I keep a backup folder of U236.1 where I can just copy it back in place.

1) First backup your game saves. They will be located in:
"C:\Program Files (x86)\Steam\userdata\*random numbers*\218620"
Also in:
"C:\Users\ <username> \AppData\Local\PAYDAY 2"

2) Follow the guide below to download the version:

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2185438382
Manifest ID will be:
3319603951769285328
Source:
https://steamdb.info/patchnotes/11113843/

So your powershell command will look something like this:
dotnet DepotDownloader.dll -app 218620 -depot 218621 -manifest 3319603951769285328 -username (your steam username) -password (your steam password)

1) Make sure PAYDAY 2 is updated on Steam

2) Close Steam completely (including in bottom right tray)

3) Rename PAYDAY 2 Steam folder to something else. This is typically located in:
C:\Program Files (x86)\Steam\steamapps\common\PAYDAY 2
May be in a different location, depending where you installed it.
You could also simply delete it if you want to save space, but it may make troubleshooting and reapplying this workaround much more time consuming. If you decide to delete it, don't forget to grab your mods out of it first.

4) Copy all the contents downloaded for U236.1 to a folder named PAYDAY 2 where the original one was.

5) Would recommend doing this later, but at this point you can copy your mods (WSOCK32.dll + mods folder) into this newly created PAYDAY 2 folder.

[Apparently this step may not be required. If anybody has tested this without this step let me know.]

Steam should still be closed at this point.

1) I usually just disconnect the PC from the network, then launch "payday2_win32_release.exe" from your newly created folder, "C:\Program Files (x86)\Steam\steamapps\common\PAYDAY 2"
This could be in a different directory, depending where you had PAYDAY 2 earlier.

2) Select "start in offline mode". The game will now launch.

3) Alt tab out of the game, connect your PC back to your network, and on Steam click "Steam" => Go Online. Now you should be able to create and join lobbies. But you will be missing many things, like community items.

4) To get those things missing, I had to close the game, and restart Steam normally, and start PAYDAY 2. Everything should now be working, including your mods. Install them if you haven't already.

Again, you may have to do this often as Steam will want you to update the game. The best thing I have personally found so far is to just make a backup of the PAYDAY 2 folder after you've installed the U236.1 + mods and copy it back in place when needed. This will require you to go back to step "Make Steam think you're using the latest version". If you want to save some time when this happens, you can copy the old PAYDAY 2 folder that we renamed at the beginning back to PAYDAY 2 - that way you don't have to re-download the game. Then once it's validated, restart from the step where you close Steam and rename the folder to something else.

If you're having problems, check the guide below:

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2185438382

Another guide for this:
https://gtm.steamproxy.vip/app/218620/discussions/8/3809529155523154229/