From the release notes for 12.1.6 MP4:
In addition, Symantec Endpoint Protection 12.1.6 MP4 is the last version to support:
■ 11.x or 12.0, which reached end of support life on January 5th, 2015. This
support includes Symantec Endpoint Protection Manager migration from 11.x
or 12.0, importing an 11.x client package, and the downloading and hosting of
definitions and other content for 11.x.
■ vShield-enabled Shared Insight Cache (VSIC) and Security Virtual Appliance
(SVA).
■ The installation of Symantec Endpoint Protection Manager on a 32-bit operating
system, or on Windows XP or Windows Server 2003.
■ A SQL Server 2005 database for Symantec Endpoint Protection Manager.
You might want to reconsider deploying SVAs in your environment. I've heard some roadmap items in regards to this but I'm not at liberty to describe anything.
Please note that this won't stop you from setting up a Network Shared Insight Cache:
https://support.symantec.com/en_US/article.TECH174123.html
Good luck!