In what many would consider overdue, Steam will no longer support the Windows XP and Windows Vista operating systems starting on January 1st 2019.
Supporting older versions of operating systems can become a development nightmare, especially when components of your application rely on third-party applications and security features of newer operating systems. Such is the case with the Steam client that already relies on on an embedded version of Chrome that no longer works on Windows XP and Windows Vista.
This causes some features such as the new Steam Chat to not work on older versions of Windows.
The Steam announcement also states that future versions of the client will require Windows features and security updates that are only available in Windows 7 or higher.
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.
It is not know what Windows features and updates Steam is referring to, but they advise all users to upgrade to newer versions of Windows in order to continue using the Steam client and all its features.
BleepingComputer has reached out to Steam for clarification regarding the Windows 7 security features and updates, but had not heard back at the time of this publication.