People don’t update because:

• They’re on an older device and can’t physically upgrade. Drop support and that’s fine they’ll still be able to use your app. Edge case: they delete it and want to reinstall…bummer
• Don’t care/don’t want to upgrade their iOS version? The most you can do is send them a push notification saying they’re missing out and drop support. Nothing much to do there. They probably aren’t the type of people who care about new features anyways

I would drop support for a old version the moment it becomes a burden to maintain and makes the code look dirty 😘