iPadOS 16 was at least updated 3 months ago (August) so there's a chance you could still get a security update if it's applicable to that version. iPadOS 15 was updated in July.
That wasn't an actual security update; that only fixed a broken toggle switch for "Advanced Data Protection" in Settings. I'm talking about patches for vulnerabilities.
iPadOS 15 hasn't been receiving the security updates for vulnerabilities that have been released for iPadOS 18.
iPadOS 15 hasn't been receiving the security updates for vulnerabilities that have been released for iPadOS 18.