MarkoH01: Also in case you did not know. Steam recently even killed the possibility to download old versions of Steam games (good to get the latest still DRM-free version i.e.) using the console. However with the help of a tool called "cheat machine" (it's free just google) you can easily revert this by simply changing some bytes in memory.
"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 I may suggest it, I'm a fan of DepotDownloader to download old Steam versions now. It's all Command Line based, but the commands aren't much different than they were when the Steam console worked, and you don't have to edit/un-edit anything to use it.