Microsoft is in the process of phasing out use of the Secure Hash Algorithm 1 (SHA-1) code-signing encryption to deliver Windows OS updates. Customers running legacy OS versions will be required to have SHA-2 support installed on their devices by July 2019. Microsoft has cited the existence of known collision attacks against SHA-1 as the main reason for advising against its use. This is only the latest step for Microsoft in its efforts to deprecate the SHA1 and older algorithms like RC4.
Source: https://threatpost.com/microsoft-updates-os-sha-1/142000/