Upgrade operating system for Symantec Appliance 5220

Our Information Security team, as per the annual audit, scanned the hosts for vulnerability and found that the OS on these appliance is very old - scan output message says, "Unsupported Unix Operating System".

Lack of support implies that no new security patches for the product will be released by the vendor. As a result, it is likely to contain security vulnerabilities.

Question:

We have upgraded the Appliance to version 2.6.1.1, however that does not upgrade the underlying OS.

Is there any technote from Symantec supporting that OS shall remain the same or plans on upgrading the Appliance OS etc. in detail?

Thanks All.