Hello cus000,
It is requested to migrate to the Latest version of SEP 12.1 RU5 immediately, as the Symantec is no longer releasing definitions for SEP 11.x
In case, you have Symantec Endpoint Protection 11.x clients in your network and cannot migrate them immediately, it is suggested to migrate the SEPM 11.x to the Latest version of SEPM 12.1.
By Migrating the Symantec Endpoint Protection Manager (SEPM) Console to 12.1, it would provide the older SEP 11.x clients definitions without any issues as there is a conversion engine.
However, it would be important to migrate the clients to the Latest version of SEP 12.1 in the near future.