The issue turned out to be unrelated to Unity IAP but isn’t the whole point of using an LTS version that it is supported long term so that you don’t have to upgrade? I have no idea if the games would be stable on newer versions and at least TextMeshPro components would have to be migrated.
Are you talking about the 2.2.8 version from this thread? Unity IAP Previous Versions (Obsolete - please see Package Manager)