I know the answer to this question depends on the situation and my target users however since I've just moved from android workspace to Swift and Xcode, I'm looking for similar android concepts over here.
In android there are platform version and distribution report which shows when I set a specific min API level, what percentage of user can use my application.
I've asked this question since I'm going to use named colors which is supported only by devices which have iOS 11.0+. I was wondering what percentage of user will miss my application if I use iOS 11.0 as deployment target. So:
Unlike Android, iOS updates are provided by Apple for free, so you do not have to wait for a phone manufacturer, and as stated in Apple's official report
80% of all devices are using iOS 12.
Having said that, I still choose a lowest possible version as deployment target, keeping in mind not putting any effort into iOS 8, with users in the low single digits. This possibility of finding the lowest version depends on features my app requires, such as if I want to use native annotation clustering, I cannot choose a version lower than iOS 11, as this feature is only available in iOS 11's MapKit update.
As of now if you support iOS 11, you will be targeting over 95% as stated on report of last 10 weeks based on mixpanel