Earlier quoted context omitted.
Safari is the new IE6, because the version of Safari you have is locked to the version of iOS you have. Features like Pointer Events, visualViewport, ES2017 etc are available on Android 4.4 , because Google still updates Chrome on old Android devices. I struggle to support iOS 9 (Safari 9) and I barely support iOS 7, because we have a number of users with old devices and we are not a consumer app so we can't just ign…
I would say you can pretty much aim for iOS 10 or even 11 as a minimum. The number of devices running iOS 7 is quite small. My moms iPad 2 runs iOS 9 and that’s about one of the oldest Idevices going
Our clients are businesses, and our users are their employees (from a wide range of backgrounds). The employees must use our web app, and our clients mostly don't get to choose what their employees use.
We log how many users are on old browsers, and we try to support users on old browsers if it isn't too costly for us to do so.
In my experience, the majority of people using a iPhone 4 or an Android 4.4 device are doing so because they can't afford to update, and they are often otherwise marginalised. I try hard not to give them yet another kick in the teeth for stuff outside their control.