Earlier diesels usually didn't have intercoolers. The Ford diesel didn't have one until 1999. The GM 6.2 didn't even have a turbo, but the 6.5s are intercooled. Some cummins have intercoolers, others don't.
Some turbos lately have been using engine coolant circulated around the bearings to cool them, but for the most part, many original turbos have a low enough stress level that they are just cooled by the oil circulating through the bearing.