If the device isn’t updated, I haven’t found the limit yet. I have 8-year-old iOS devices on original iOS versions and battery life is like-new. There is probably a limit. I just haven’t found it yet, and I reckon it would exceed the usability of the iOS version itself.
What do I mean by this? iOS 10 will probably be incompatible with my own requirements before the battery dies. It is already, actually.
My almost 5-year-old iPhone Xʀ on iOS 12 gives me, today, 18 hours of SOT. Battery health is irrelevant if the device isn’t updated. Like I said, iOS 12 will be useless before the battery on that Xʀ on iOS 12 gives me problems. For all intents and purposes, batteries on original iOS versions never die.
