Earlier this year, Valve announced that the start of January 1, 2019 will be supported for Windows XP and Vista. Anyone who stays on XP or Vista should expect to be new Windows 7 or later if they want to use Steam.
The decision was made to continue delivering Steam gamers to these platforms, with less than 1% of each user base coming from these two operating systems. However, this is not the only reason behind this decision. Valve said in a statement that "Steam's latest features on a built-in version of Google Chrome will no longer work on older versions of Windows," like Windows XP and Vista. Here is the full announcement as in June returned (via MSPU):
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Beginning January 1, 2019, Steam will officially quit Windows XP and Windows Vista support. This means that after that the Steam Client will no longer run on these versions of Windows. To send steam and use any game or other products purchased through Steam, users must change an updated version of Windows.
The latest features in Steam are based on a Google Chrome version, which no longer works on older versions of Windows. In addition to future versions of Steam, Windows features and security updates only request in Windows 7 and above.
For the rest of 2018, Steam will continue to play on Windows XP and Windows Vista, but other features in Steam will be limited. So, new features like the new Steam Chat can't be available. We encourage all users on these operating systems to improve on new versions of Windows to access the latest features of Steam, and to get access to all games and other content of steam.
Windows XP and Vista are making another battle this year with Antivirus maker, Avast, a similar anonymous one. As a note, Avast security products will officially quit by updating for Windows XP and Windows Vista operating system early in the morning. However, the company will continue to send virus-definition updates.