Starting today, Valve's Steam distribution platform is no longer supporting Windows XP and Windows Vista. In order to continue to use Steam, and any products purchased through Steam, users will need to upgrade to a more modern version of Windows.
Support is getting dropped due to a variety of reasons. First up, XP and Vista are getting very old, and are no longer used by a large percentage of gamers. They're both obsolete and no longer receiving security updates, so nobody should still be using these operating systems on Internet-connected machines. Secondly, Valve no longer wants to develop a separate version of Steam. The latest versions of Steam use an embedded version of Google Chrome, which isn't available for older versions of Windows.
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.
On a related note, anti-virus company Avast also dropped support for Windows XP and Vista at the end of 2018. These users of Avast will no longer receive feature updates, but at least at this moment, they'll still receive virus definition updates.