It turns out that older iPhone models like the 6S can run iOS 15, but only the iPhone XS and newer can take full advantage of all its features thanks to the A12 chip.
iOS 15 – Every Feature That Doesn't Work on Older iPhones
Apple confirms that iOS 15 will run on everything from an iPhone 6S (2015) to the new iPhone 12 (2020), but only the iPhone XS (2018) and later will have access to all features. Many of the processor-intensive features of iOS 15, and those that will run on-device instead of requiring access to Apple servers, simply won't work on older devices.
It comes down to the A12 processor, which debuted in the iPhone XS, iPhone XS Max and iPhone XR. As Pocket-lint points out, some features new to iOS 15 will work on older devices, but not everything. This hardware limitation prevents those with iPhones older than a 2018 model from using 100% of what iOS 15 has to offer.
Some features that aren’t available on older iPhone models include Portrait mode and spatial audio for FaceTime, support for digital keys, and some pretty real-time imagery for the Weather app. A few supposedly memory-intensive Apple Maps features are also gone, including AR-view walking directions and added map detail (e.g. trees, landmarks, etc.).
