The bar crept forward. 50%. 80%. 90%. This was the moment of truth. Often, old IPA files would fail verification with Apple’s servers, throwing up a cryptic error code that meant "this software is too old to live."
Restoring service to older hardware that cannot run the modern YouTube app. How to Install YouTube 13.42.6
Since Apple does not allow downgrading via the App Store, you must the IPA. Sideloading installs an app outside the official store, but it comes with one limitation: the app must be re-signed every 7 days (unless you have a developer account).