AFAIK, it may need to verify game files, maybe download one or two additional small files (which I thinks is the specific manifest) and be good to go. Another thing I’m reading just on a brief search is you can find the games steam id and then modify the appmanifest_####.acf file to read-only to prevent any further updates if the option does not appear in properties. Good luck, and don’t give up hope!
Edit to add: the commenter who gave this bit of info also rightly noted you should back up your game files if you have the disk space to do so
If you downgrade does it need to install that version? Or does it just say “ok it’s already on here let’s launch”?
AFAIK, it may need to verify game files, maybe download one or two additional small files (which I thinks is the specific manifest) and be good to go. Another thing I’m reading just on a brief search is you can find the games steam id and then modify the appmanifest_####.acf file to read-only to prevent any further updates if the option does not appear in properties. Good luck, and don’t give up hope!
Edit to add: the commenter who gave this bit of info also rightly noted you should back up your game files if you have the disk space to do so