Now you can steam no longer used on older operating systems. The support is set for January 1, 2019 as valve announced.
This step should not be particularly surprising. The company explained the explanation a while ago in the light of decreasing user numbers einzuschrnken.
Decreasing users, decreasing support
Support has been terminated for the following operating systems:
macOS 10.7 (Lion)
macOS 10.8 (Mountain Lion)
macOS 10.9 (Mavericks)
macOS 10.10 (Yosemite)
The number of users is negligible in the mentioned operating systems. In the latest hardware survey, Windows XP comes to 0.11 percent, Windows Vista at less than 0.05 percent.
If you want to continue using the Steam client and its games, upgrade to a newer version of Windows or Mac. Valve recommends the use of Windows 10 or macOS 10.14.