* There's an app in the Universe repository, which consists of software that is not formally supported by Canonical at all, which has security updates that are actually available from Canonical if you're an Ubuntu Pro subscriber.
This is Canonical getting their hands dirty and applying their own patches faster than the package maintainers. If the package maintainer release a patch then that is applied like normal, no Ubuntu Pro required.
Once again: If they weren't offering this, it wouldn't be patched at all.
If they didn't have this, the security would be lower for everyone.
They have not taken anything away from anyone here, they have only provided extra services to people who pay.
They communicate this poorly, obviously. And it needs to be made clear that they are not withholding community patches, only contributing for a price priority patches to previously community-only packages.
* You're running a version of Ubuntu that's still supported.
* One of the packages installed on your system has a known security vulnerability.
* There's an updated version of the package available that fixes the vulnerability.
* Since you're not subscribed to Ubuntu Pro, you're not allowed to update to the fixed version.