In one sentence: If you can run iOS 14 you can run iOS 15. The iOS 15 device support list includes iPhone 6s and later.
iOS 15 Device Support List
- iPhone 6S, iPhone 6S Plus
- iPhone SE (2016), iPhone SE (2020)
- iPhone 7, iPhone 7 Plus
- iPhone 8, iPhone 8 Plus
- iPhone X
- iPhone XR
- iPhone XS, iPhone XS Max
- iPhone 11, iPhone 11 Pro, iPhone 11 Pro Max
- iPhone 12, iPhone 12 mini, iPhone 12 Pro, iPhone 12 Pro Max
- iPod Touch (seventh generation)
Some features, like portrait mode in FaceTime, requires an iPhone with an A12 Bionic chip or newer. This means the iPhone XS or newer.
Shouldn’t the 7th gen iPod Touch, which can run iOS 14, be on this list?