Support for two former operating systems, XP and Vista will no longer be in effect after Jan 1st 2019. This is due to embedded software within steam becoming incompatible with the old os's:
Ref:
Windows XP and Windows Vista Support - Steam - Knowledge Base - Steam Support
community
compatibility
end support
game client
gaming
google chrome
online gaming
operating systems
security updates
software updates
steamsteam chat
steamfeatures
support
technology
upgrade
user access
windows vista
windows xp
