As reported here #9245 (comment)
ServiceChannelProxy isn't a public type but it's listed in the docs as such. In the ServiceModel package there are a ref assembly and an implementation assembly. It's public in the implementation assembly as it needs (or needed I think from back in the UAP/UWP days) to work with DispatchProxy, but it's never been in the reference assembly. So we need to figure out why it is showing up in MS Docs.
cc @mconnew