OSes which are safe against Meltdown on Mainstream Support:
macOS 10.13.2

Windows 10 Build 16299.192

Windows 10 Build 15063.850

Windows 10 Build 14393.2007

Windows 8.1 latest KB on MS Download Catalog

Any Linux Distro using kernel 4.15

Extended Support protected OSes:

Windows 7 SP1 latest KB on MS Download Catalog

Windows 10 Build 10586.1356 (LTSB only)

Windows 10 Build 10240.17738 (LTSB only)