The specific addition that the documentation says to have been made is remove(atOffsets offsets: IndexSet). There also is a new method: applying(_ difference: CollectionDifference<Self.Element>) -> Self?. These changes were made in Catalina Beta 5.
The question raised on the other thread is whether these changes actually are changes to the Standard Library, as the documentation indicates, or whether they might be extensions made within the SwiftUI framework.