MacBooks and iMacs running macOS 10.10 Yosemite will no longer be supported on the Steam platform from January 1, 2019, according to Valve Corp.
The reason for support being dropped is that Steam now uses Google Chrome as an embedded version, something that older macOS versions do not support.
Specifically, support will cease for macOS 10.7 Lion up to macOS 0.10 Yosemite. Players will need to upgrade to at least macOS 10.11 El Capitan in order to play their purchased games on an Apple computer.
As of now, the new version of Steam Chat will not work on the older macOS versions, and the Steam platform itself will be limited in functionality on such devices.
If you’re on Steam and have purchased any games in the past, we recommend upgrading to macOS 10.11 El Capitan or higher to continue enjoying the platform’s full suite of offerings.
Here’s the full announcement from Valve:
“Starting on January 1 2019, Steam will officially stop supporting macOS versions 10.7 (“Lion”), 10.8 (“Mountain Lion”), 10.9 (“Mavericks”) and 10.10 (“Yosemite”). This means that after that date the Steam Client will no longer run on those versions of macOS. In order to continue running Steam and any games or other products purchased through Steam, users will need to update to a more recent version of macOS.
The newest features in Steam rely on an embedded version of Google Chrome, which no longer functions on older versions of macOS. In addition, future versions of Steam will require macOS feature and security updates only present in macOS 10.11 (“El Capitan”) and above.
For the remainder of 2018 Steam will continue to run and to launch games on these versions, but other functionality in Steam will be somewhat limited. For example, new features such as the new Steam Chat will not be available. We encourage all users on these operating systems to upgrade to newer versions of macOS in order to have ongoing access to the latest features of Steam, and to ensure future access to all games and other Steam content.
Users of macOS versions 10.7 (“Lion”) through 10.10 (“Yosemite”) can find more information on upgrading macOS here.”