Forced air ductwork is rarely insulated,
Maybe 30 years ago, but these days all new ductwork is insulated, usually with an inch or more of fiberglass.
This house was built in '41 and the heated side of the ductwork was simply wrapped in asbestos tape. The unheated side was just bare metal. When I weatherized the house one of the things I did was to use non-asbestos fiberglass/mastic tape to seal all the seams (some of which had worked loose and were leaking), and then cover all the ductwork with foil faced fiberglass 1" insulation, the foil side out. Then I sealed all the joins in the foil covering with foil tape. This effectively insulated the ducts as well as limited the amount of asbestos that might later otherwise become airborne. I also added the same 1" insulation around the return ductwork. Not the best application but I figured it could help keep things warmer between cycles.
The area under the master bed/bath addition already had sloppily insulated ducts. My possible mistake was in not removing that old fiberglass insulation, sealing the seams, and then insulating it. Instead I just installed insulation over the old stuff. It was sealed with foil tape so any leaks probably don't get very far, but I'm probably someday going to remove the outer insulation, seal all the seams, and then re-install the insulation. When I get a round toit.
PS-After I weatherized/insulated the house and sealed/insulated the ductwork, I noticed an immediate difference in the amount of time the furnace would fire up on cold days. Instead of being on for an hour or more, it would come on for only 30 minutes or less on the coldest days. The gas bills (actually number of therms consumed) dropped proportionately - a nice result.
One benefit of forced air systems, not mentioned so far, is that with good filtration it can actually clean up the house air a fair amount. The best filters even remove cooking odors and smoke. In tightly sealed homes, this could be an important factor.