https://techcrunch.com/2018/01/23/apple-issues-meltdown-fix-for-macs-running-sierra-and-el-capitan/

A wise person once said, “don’t upgrade to the latest OS X until it’s been patched a few times. And even then, sometimes it isn’t really worth it, though maybe for security stuff.” These words (from before the name change to macOS) have inspired many to stay on older versions of the OS, though lately that meant remaining vulnerable to Meltdown while those on the latest version were protected. Fortunately Apple has brought Sierra and El Capitan into the fix fold.

The latest batch of security updates fix a few random exploits here and there, but the marquee feature is definitely closing the Meltdown vulnerability on Sierra and El Capitan Macs. If you’re running one of those, and who can blame you, you should upgrade as soon as possible.