ServiceDesk is basically made to work with NS7; it's delivered via an NS7 solution, it's components were built with NS7 in mind, etc. However, this doesn't mean that the old Helpdesk components (or any components coded for NS6) "went away".
When you inStall SD7, it'll ask for an NS7 as a licensing server. This actually creates an entry in your credentials tool and makes it the default. Once the install is done, you can register an NS6, and get the Helpdesk components to appear in the tree.