Specifically, Valve says that Steam will stop working with macOS versions 10.7 (“Lion”), 10.8 (“Mountain Lion”), 10.9 (“Mavericks”) and 10.10 (“Yosemite”).
The change is due to features in Steam relying on an embedded version of Google Chrome, which no longer functions on older versions of macOS. Valve has announced that beginning on January 1st, 2019, Steam will officially drop support for macOS 10.10 Yosemite and earlier, dating back to Lion.
