( Found by @Acethedolphinguy )

Where to download them: https://steamdb.info/depot/229581/manifests/

You have to type in the "download_depot 229580 229581 XXXX" with the ID of the version you want to download replacing the "XXXX"

How to Install: https://steamcommunity.com/sharedfiles/filedetails/?id=1805934840

1. Run Cheat Engine.
2. File -> Open Process -> Steam
3. Set the right-hand settings to the following:

• "Memory Scan Options": steamclient.dll (just keep pressing S when the drop-down is open to iterate through each and find it quicker)
• "Value Type": Array of byte
• "Hex": checked
• "Writable": unchecked (!)
• "Array of byte": D0 84 C0 0F 85 2E FF FF FF

4. Click "First Scan". You should get a single Address and the searched Value next to it.
5. Right-click the Value -> Change value of selected address(es)
6. Enter new value: D0 84 C0 E9 2F FF FF FF 90
7. Click OK. Done. You can now exit Cheat Engine and are able to downgrade games without the "Manifest not available" error, until you exit Steam. Continue through the guide as before.

You will have to do this every time Steam is (re)started

If anything isn't clear (it isn't really for me), just ask and hopefully someone who understands this stuff better than I do can help. I'm just putting this here because now we know how to do it. ^^;

Though currently, none of the previous versions are allowed (they're all unfinished anyway so running the rules we currently have isn't possible), that might change one day, and having this here as a resource would be nice for when that does happen.