In general, not all iPhone devices are able to upgrade to the latest version of iOS. The ability to upgrade to a new version of iOS depends on the hardware capabilities of the device and whether the device is still being supported by Apple.
Below is a list of iPhone devices that are not able to upgrade to iOS 15:
- iPhone 5
- iPhone 5c
- iPhone 6
- iPhone 6 Plus
Note that this list is not exhaustive and only includes some of the older iPhone models that are not able to upgrade to iOS 15. If you have a different iPhone model and are unsure whether it is able to upgrade to iOS 15, you can check the list of devices that are compatible with iOS 15 on Apple’s website or contact Apple for more information.