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. Users will need to update to a more recent version of Windows to launch any games.
The new features in Steam, namely chat, relies on an embedded version of Google Chrome, and Chrome has stopped supporting windows Xp forcing steams hand in this regard due to their reliance on this.
Steam (and we) encourages 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. If you’re a rebel – Steam support in Linux has also improved drastically during 2018 and might be a great option for those few that remain steadfast in their hate of anything Aero or using Tiles.