Hm, which kind of backup/tool do you made?
Back in the days in iTunes there was this “mobile applications” folder.
But now …
I made backups via iMazing. But those Backups did not include the .ipas.
I downloaded the old app directly from the app store thru Apple Configurator. It can sync apps to the phone directly. For that, it temporarily downloads a copy and then pushes it to the phone. I just made a backup of the app while it was in the temp folder. You can google how to backup apps using Apple Configurator.
Obviously, you can only backup the version that was in the app store at the time this way, meaning, now it will download the 2.0 version instead of the classic one.
ahh, understand. thanx for the information. good to know for future issues. 
Please let me know if anyone knows how to restore the old version Clear 