Is iPhone 6 compatible with iOS 16?

The iPhone 6 can only be updated to iOS 12.5.7 and the iPhone 6s iOS 15.8.3. You will not be able to update to iOS 16, as it requires an iPhone 8 or later. Since you should start looking into a newer iPhone that can run a more up to date iOS, I would suggest an iPhone 15 Pro or later as it supports Apple Intelligence and the newest iOS 18.

iPhone 6 iOS 12.5.7 is the latest version and iPhone 6s iOS 15.8.3 is the latest version boths iPhones can’t be update beyond those iOS versions

If you want to run iOS 16, you will need an iPhone 8, iPhone 8 Plus or iPhone X.