This is a custom XML comments element implemented by the Sandcastle Help File Builder. It will not appear in the list of valid elements for XML comments IntelliSense.
This element has been deprecated in favor of using the project's API Filter project property to exclude members. The help file builder will translate members with this element into an API filter entry. Be aware that if used on an overloaded member, all versions of the overloaded member will be removed from the documentation due to the way the API filter works in Sandcastle.