The push from MS to shut down usage of all the old versions of Windows goes much deeper than support issues... after all, in addition to age, and flaws, There is the biggest snoop/snitch at Microsoft's disposal: Cortana. Microsoft wants you on "the latest and greatest," where, by default with a one finger salute to the users and their privacy, Cortana tells all to Redmond. This still in spite of all the "improvements" to privacy controls. Yeah, that isn't exactly on topic...
I can understand the Steam move, as XP and Vista won't support the newest versions of the browser, and privacy and security laws are mandating certain security support features only found in the newest/newer browsers. THIS affects Steam as well since you can make purchases through the steam app.