Valve has announced that Steam will officially stop supporting the Windows XP and Windows Vista operating systems on January 1st, 2019. What this basically means is that the Steam Client will no longer run on those versions of Windows after that date and as such you won’t have access to your older games that could – theoretically – still run on these operating systems.
As Valve stated:
“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.”
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.
Valve concluded by encouraging 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.