Steam made an announcement months ago that they will halt support for Windows XP and Vista on January 1st, 2019. So, today is the date, and Steam no longer supports Windows XP and Vista because they can’t support a lot of Steam’s new features.
Steam stated the following.
They encourage users in their post to upgrade to a new version of an operating system as it can’t support the new Steam UI and many more features. If you are using Windows XP or Vista and you use Steam, then you should upgrade to Windows 7 and upwards.
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