Not sure what happened, but it was not intentional to remove gcs, but I can see that it is missing on my machine as well. It should be in the Duplicati.Library.Backend.GoogleServices.dll library, but it is not loaded for some reason.
If you directly installed 126.96.36.199 through an installer, then that is now your “base version” which is as far back as you can downgrade without “over-installing” from an older installer. You can check your base version by looking at the BaseVersionName line on the main “About” -> “System info” page.
If you updated to 188.8.131.52 using the GUI updater, then you should be able to downgrade using the steps described here: