The reason for the upcoming change is due to a change in Google Chrome which has ceased support for older systems however planned future changes to Steam will also apparently require modern Windows security and feature updates so XP and Vista were bound to be dropped eventually anyway.
Windows XP and Vista users will still be able to buy and play games via Steam for the rest of 2018 however they won't have access to any new features that are launched before then.
Here's the full statement from Valve.
Starting on January 1 2019, Steam will officially stop supporting the Windows XP and Windows Vista operating systems. This means that after that date the Steam Client will no longer run on those versions of Windows. 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 Windows.
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.
For the remainder of 2018 Steam will continue to run and to launch games on Windows XP and Windows Vista, 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 Windows in order to have ongoing access to the latest features of Steam, and to ensure future access to all games and other Steam content.
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