For the most part just like iOS devices, Apple’s macOS devices tend to be pretty well supported. For example my 7-year old 2011 MacBook Air has been on the receiving end of major OS updates right up until macOS High Sierra, which was the last major update my laptop was capable of receiving. That’s a pretty long period of time for support.
Unfortunately if for whatever reason you’re running on macOS 10.10 Yosemite (which was released in 2014) and you rely on Steam quite a bit to play games, it seems that come January 1, 2019, Steam will no longer be supported on your computer. According to Valve’s announcement, it seems that this has to do with a feature in Steam that relies on a version of Google Chrome that no longer works on older macOS computers.
“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.” Valve also notes that while Steam will continue to run until the cut-off date, certain functions will be limited or not available entire, such as the new Steam Chat.
To be fair Steam has supported macOS computers for the longest time ever, since macOS Lion which was released in 2011 (also known as OS X 10.7 Lion), so we suppose it was only a matter of time before they dropped support for older versions of the OS.