Besides the iOS 16 release, there is also a security update to iOS 15.7 for older devices. This should make it clear that Apple will continue to maintain the older system.
All owners of an iPhone 7 and 6s as well as an iPad Air 2 and newer, iPad 5 and newer and finally the iPad mini 4 and newer can look forward to the update. The seventh generation iPod touch was also considered.
This means that you can continue to use older devices safely, as Apple is already doing with security updates for devices with iOS 12.