You have a system that has long been out of maintenance. Although you might have implemented up to latest available patch and everything’s running smoothly, there is no beating around the bush: Your system is insecure.
The software vendors as well as security researchers will have found security vulnerabilities that are getting fixed in higher releases, but since your system is out of maintenance, there is no patch available. And worse you’re not even made aware that there is a problem. You’re probably not even realizing that components that exist in your system can be exploited, because the relevant security patches are classified as applicable to a higher release.
You only have three options here: Either you manually scan through the security patches for higher release versions of your systems and evaluate if your system is vulnerable too. If it’s only a port that you have to close or some configuration settings to do, that might be easy to accomplish. But more likely you’re in for option 2. Upgrade your system to the higher release and pay maintenance again. Or you could directly go to option 3: shut down this insecure system.
Because the fact remains: Old systems that are out of maintenance are insecure.