This isn't true. You can still install existing IPAs you have saved in the past by syncing it with Finder. You can also just AirDrop an IPA to your iOS device to install it.
> Now, the only way to keep your software on your iPhone indefinitely is to never delete it, and never reformat your phone.
You can still back up IPA installers by downloading them with Apple Configurator 2. https://ios.gadgethacks.com/how-to/download-ipa-files-for-io...