Apple's press release and preview page say that the OS will support "iPhone 8 and later." That means that the new update won't be compatible with phones using Apple A9 or A10 chips, including the iPhone 6S, 6S Plus, 7, 7 Plus, the first-gen iPhone SE, and the seventh-generation iPod Touch.

iOS 16 official list of compatible devices:
iPhone 8 and 8 Plus
iPhone X, XR, and XS series
iPhone 11 and 11 Pro series
iPhone 12 and 12 Pro series
iPhone 13 and 13 Pro series
iPhone SE (second generation) and newer