If they want to use an old phone, they can just stop updating their apps and not installing new apps. I still own a iPad 3 with the same, old, software stack from 2013. It still works, I can do things. I just don't expect todays developers to make a herculean effort so that I can run their apps on my old device. It is not just the overhead of supporting multiple different OS versions with different technologies, it i…
No, you cannot. Apple does not give you the freedom to use old versions of software. You can't downgrade your iOS version. You can only upgrade apps, and iOS only lets you download the latest available version of an app for your version of iOS. Need to do a device reset? If you have managed to keep any apps on old versions, you'll get upgraded now. And you can't even save APKs and sideload because of Apple's develope…
Not only can you use old versions of software, you can download them again if you reset your device.
Visit the purchased tab in the App Store and select the app you wish to download again, and you will receive the last compatible version for that particular device.