iPhone models to support

Any iPhone as iPhone 6 / 6 Plus or below does not support iOS 15. They support up to iOS 12.

Any iPhone from iPhone 6s / 6s Plus and above do support iOS 15.

So setting deployment target as iOS 13, iOS 14, or iOS 15, is equivalent with respect to your question.

If you can, you should probably support at least iOS 13.

This is centrally your decision. Considering this support question more generally, I’d be inclined to pick a smaller subset of (newer) that can be tested and supported and that can run your app with adequate performance and features, and then expand that list over time as warranted.

If you decide to support them, you’ll want to test with older and smaller (and slower) iPhone models, just so you know what your app looks like on those.

iOS 13, iOS 14, and iOS 15 do require iPhone 6s and newer. The features available with iOS 13, iOS 14, and iOS 15 further depend on the particular iPhone model. Older iPhone models such as iPhone 6s—though supported—lack access to certain newer features of iOS 13, iOS 14, and iOS 15. For details on these older models and associated feature limits, scan these footnotes.

Though it’s entirely your call, I wouldn’t spend all that much time with iOS 13 and iOS 14 support (to start with), as those versions tend to be used by folks either with insufficient storage, or with lackluster update practices. Folks that can potentially be support problems awaiting, in other words.

As for other considerations you might not have pondered, maybe also iPad, and maybe what your app looks like on Mac M1—if you want to enable that for your app.

With the upcoming WWDC, we’ll likely know more about what Apple plans for iOS support for the various models.

Again, your call, and your trade-offs.