WN19-CC-000260 - Windows Server 2019 Windows Update must not obtain updates from other PCs on the Internet.
Windows Update can obtain updates from additional sources instead of Microsoft. In addition to Microsoft, updates can be obtained from and sent to PCs on the local network as well as on the Internet. This is part of the Windows Update trusted process, however to minimize outside exposure, obtaining updates from or sending to systems on the Internet must be prevented.
Configure the policy value for Computer Configuration >> Administrative Templates >> Windows Components >> Delivery Optimization >> 'Download Mode' to 'Enabled' with any option except 'Internet' selected. Acceptable selections include: Bypass (100) Group (2) HTTP only (0) LAN (1) Simple (99)