Make app available only for iPhone 6S and above

Have a look at the Device Capabilities Table.

As you can see there, when you use ARKit, your app will automatically be restricted to iPhone SE, iPhone 6s and higher. Older iPhones will be unable to download your app from the App Store. But be aware that there is no way to opt out from iPhone SE, so you have to ensure that it works there as well.

Unfortunately the only way to restrict support of your apps on the App Store to certain devices, is to either set the respective capabilities from the table linked above, and/or by raising the minimum required iOS version (deployment target). Also be aware that you can only add capablities, you cannot remove them (with a subsequent binary version for example).

Many developers out there would like to have a more fine graded control of what devices are supported exactly. Apple's capability system works fine for some certain simple cases. But there are many use cases where this system is insufficient. For example: let's say your app requires a certain min. RAM size, and then even for a universal binary. Or let's say you have an app where you cannot push all required controls into a tiny screen size of an iPhone SE (or smaller).

In practice many developers simply write in the App Store Description: "Following devices are supported: ... ". Probably not what Apple had in mind.

The store will warn, however, also see Table 6 Dictionary keys for the

UIRequiredDeviceCapabilities

UIRequiredDeviceCapabilities