iOS 12 was the last release for the 6, which was the last phone cut off from updates (and they sold it for an especially long time). The 6s and 7 can update to

iOS 12 was the last release for the 6, which was the last phone cut off from updates (and they sold it for an especially long time). The 6s and 7 can update to iOS 15, which is still getting security updates.