This comes roughly a month after Microsoft officially ended support for Windows 10 version 1511, which was first released to PCs in November 2015. The move is part of Microsoft's larger "Windows as a Service" (WaaS) model, under which new major feature releases are flighted to the public at a more rapid pace. As a result, each major release is only supported for a limited time.
There was speculation that support may have been extended, as Microsoft pushed out a small cumulative update to version 1511 weeks after support had officially been dropped. If you're running Windows 10 Enterprise or Education, you should continue to receive updates to version 1511 through April 2018. Anyone running Windows 10 Home or Pro should upgrade to the latest version of Windows 10.
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