To be fair to Apple, the company doesn’t benefit financially in any way from you installing the latest version of iOS (or macOS, though that’s a somewhat different issue for major upgrades on older hardware that’s being held at an older version intentionally), so I don’t see notification of available upgrades as advertising.
It would be negligent of Apple not to inform all users of operating system upgrades that include important security fixes, which is nearly all of them. You can choose not to install them and risk not being protected against increasingly public vulnerabilities, but the notifications are reasonable.