This is according to the support page on Steam’s website in which according to Valve, “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.”
Like we said, Windows XP and Windows Vista are pretty old and are at least 3-4 versions old and are succeeded by the likes of Windows 7, 8, and 10, so it’s actually more surprising than anything that Valve has supported either versions for as long as they have. While support will end in 2019, Valve also notes that functionality of Steam on either version of Windows will be limited, such as the newly announced Steam Chat and its features.