Apple released updates for all iOS devices back to the iPhone 5s, that phone is more than 12 years old and is still getting security updates.
I’m on the iPhone X and trying to buy European for my next phone, especially with Tim Apple sucking up to Trump, but 12 years of security updates is impressive commitment.
The official Apple release pages have some general details, and there’s a writeup on Ars Technica tha’s based off of Apple’s published info.