Valve revealed this week that it plans to drop Steam's support for Windows XP and Windows Vista in 2019. Starting on January 1, the Steam client will no longer work on PCs running either of Microsoft's aging operating systems (via Neowin).
According to Valve's support article, the move comes down to a combination of factors. First, Steam uses an embedded version of Google Chrome, which no longer works with XP or Vista. Secondly, the Steam client itself will require security features that only exist in Windows 7 and newer. From Valve:
The newest features in Steam rely on an embedded version of Google Chrome, which no longer functions on older versions of Windows. In addition, future versions of Steam will require Windows feature and security updates only present in Windows 7 and above.
Until the January 1 cutoff, Steam will continue to work just fine on Windows XP and Vista. However, Valve notes that some features, like the upcoming Steam Chat, won't be available.
Still, if you're still running an older machine as a gaming rig, you'll want to make sure to make the move to Windows 7, 8.1, or 10 by the end of the year.