https://wccftech.com/apple-will-not-force-update-you-to-ios-15-and-ipados-15-you-will-continue-receiving-security-update-on-older-firmware/

Here's something really interesting - Apple will not force you to update to iOS 15 and iPadOS 15 when it becomes later this year at all.

By default, iOS and iPadOS updates download and install automatically overnight. Some users will argue that Apple forces you onto a newer firmware update. But, with iOS 15 and iPadOS 15, you can actually opt of that completely.