Safe to put old white paper-backed fiberglass insulation on metal ductwork?
All fiberglass insulation is white and dyed a colour (so there's no functional distinction among colours).  The melting point of fiberglass is about 1,300 F. But the paper backing is, obviously, not so flame retardant.  Yes, foil-backed insulation is better in every respect but batt insulation (minus the paper backing) will function.  BTW, foil-backed bubble wrap should never be used (and for the cost and claims, is a pseudo-fake product) ...if it's still on the market. 

Fiberglass batt insulation works best if not compacted (ie, by being taped tightly with duct tape...which begins to melt at 200 F w/ toxic smoke).  There's also a safer product specifically for taping ductwork and duct insulation that is made of metal foil.