Hi there,
Yes, you can disable the Default WSUS Website and it seems it doesn't have much effort on WSUS services. It also depends upon the number of servers you have.
Disabling default web page on WSUS server https://social.technet.microsoft.com/Forums/ie/en-US/5070d56a-2746-401c-84a4-30dd8ccb82da/disabling-default-web-page-on-wsus-server?forum=winserverwsus
If your network is "air-gapped"--if it does not have access to the internet at all--you can still use WSUS to provide updates to client computers on the network.
If you only have one WSUS server, it must have internet access, because it needs to download updates from Microsoft. If you have multiple WSUS servers, only one server needs internet access. The others only need network access to the internet-connected WSUS server. Your client computers don't need internet access; they only need network access to a WSUS server.
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