Developers are most excited to kill off support for older platform, while business people are not. So how do you decide?
App Store Numbers
App Store provides the uptake numbers.
This is as of Mar 2022, when the latest iOS 15 has been released for 6 months.
By supporting iOS 14 (n-1), you would have supported 93% of all iPhone devices.
App Store Connect
You can also use App Store Connect analytics and filter the platform versions to have a sense how many % of your users are using the older platform versions.
As you can see, for this particular app of mine, iOS 13 is no more than 0.5% of total active users.
Rule of thumb
Support at least n-1 (latest and previous, eg. iOS 15 & 14).
Support n-2 to be very supportive.
Balance with the technical benefits.