We have a subdivision along our river, and in those homes code required mechanical equipment to be elevated to some degree, and it also required ductwork to be above the flood plain (which is just silly, since the entire dang house was below the flood plain).
In other areas, furnaces are often installed on a plenum (i.e. the metal box that serves as the termination point for the return ducting). Those plenums are generally 12" tall or more.
, there is no other requirement for a furnace to be on a platform. We've installed many directly on the concrete basement floor, and the inspectors pass them every time.
Also, different regions have differet established practices, and that may be just one of those practices. For example, in our region you almost never see ductboard in either residential or commercial. However, the RUUD tech rep here reported that another region about 85 miles away uses a LOT of ductboard in residentail apps. Go figure ...