Ungrounded deltas exist in many places where people install stepdown transformers backwards to feed equipment that does not match the service voltage.
Such as a site that has a center tapped 240V delta service and needs a feeder for some 480V equipment. The 480V winding will be a pure delta, and has no way to ground it, other than make it a corner grounded. They usually know less about corner grounding requirements than they do ungrounded systems.
208 to 480V, 240V to 480V, or 480 to 600V, proper step-up transformers exist, but many times people use the wrong one in reverse because its already on hand or much cheaper to find a used one.
I see it often in the plants that I work in, and never see a ground fault monitoring system.