To protect your security, Windows operating system updates are dual-signed using both the SHA-1 and SHA-2 hash algorithms to authenticate that updates come directly from Microsoft and were not tampered with during delivery. Due to weaknesses in the SHA-1 algorithm and to align to industry standards Microsoft will only sign Windows updates using the more secure SHA-2 algorithm exclusively.
Customers running legacy OS versions (Windows 7 SP1, Windows Server 2008 R2 SP1 and Windows Server 2008 SP2) will be required to have SHA-2 code signing support installed on their devices by July 2019. Any devices without SHA-2 support will not be offered Windows updates after July 2019. To help prepare you for this change, we will release support for SHA-2 signing in 2019. Some older versions of Windows Server Update Services (WSUS) will also receive SHA-2 support to properly deliver SHA-2 signed updates. Refer to the Product Updates section for the migration timeline.
Reference: KB 4472027 contains more information