https://en.wikipedia.org/wiki/Navistar_DT_engine. 

They stopped producing mechanically injected DT engines in 95. But I know chassis manufactures still used them for a while after, I've seen them :P. So its probably pretty rare to find one in a later chassis, but they were built to order for a while and had lagging emissions standards compared to light duty. 

A healthy Cummins 4B/T, 6B/T engines and even the DT360s generally weren't equipped with heating systems unless they were in for on-road applications. The major issue with the cold is #2 will gel up and requires mixing with #1 (kerosene) and can usually start up fine under 0*f. Starting aids were mostly added for emissions arguably. Many engines need starting aids, like International V8s, and later 4 stroke Detroits. As the engines wear though, they lose compression and injectors under-perfom and starting aids are helpful and can be required after a certain point under freezing.