Is there a need for reference plane for differential traces
It is not entirely clear if the question is asking about traces on a PCB or wires in a cable system, and the answer is different for those two cases.

Assuming it is actually required for the signal path to maintain correct differential impedance, then on a PCB, this will only be practical when the traces are routed over a plane or between two planes.

The reason is that edge-coupled traces do not couple to each other very well, so in reality they will be coupled to each other only through the ground plane. Two 50 Ohm traces next to each other have a differential impedance of around 100 Ohms, or maybe a bit less. To the extent that the differential impedance is less than 100 Ohms, it is due to mutual coupling. I suggest interested parties find an online trace impedance calculator and play around with some geometries. Start with 5 mil microstrip with 2.5 mils of dielectric separating trace from ground plane. If you use metric, then use 0.14 mm trace width and 0.07mm dielectric.

Once the traces transition off-board into a cable system, it is possible for them to maintain their correct impedance in the cable without a shield, as long as the cable is designed correctly.

So this is a long-winded way of saying that if it is important for the traces to maintain correct impedance on the PCB, they will have to be routed over a plane. But in the cable, they will likely be able to maintain correct impedance without a shield.

This is an old question. I chose to answer it anyway. Probably the OP is long gone but maybe someone else will benefit from the answer.