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
endsupport
game client
gaming
google chrome
online gaming
operating systems
security updates
software updates
steam
steam chat
steam features
support
technology
upgrade
user access
windows vista
windows xp
M$ has stopped selling retail copies of Windows 7. They know that companies don't want that Windows H8 crap and they want to try to force people hands by ending support for XP. Do they really think companies are going to switch to H8? I'm thinking you start seeing Apple and Android sales going...