Apple released supplemental security updates for older versions of the Mac OS X operating system. These updates backported the company’s Meltdown and Spectre patches, originally released in December 2017. Apple also released security fixes for iOS 11.2, Safari 11.0.2 and tvOS 11.1.2. Experts now expect the company to backport the Spectre flaw to older MacOS versions in a future update as well. The company also released a fix for a zero-day released at the end of last year by a security researcher.
Source: https://www.bleepingcomputer.com/news/apple/apple-backports-meltdown-patch-to-older-macos-versions/