whats interesting is why the question is so specific, i.e. rear antirollbar rather than just antiroll bars.
possibly we're looking at a specific case in question, certainly on a front wheel drive car fitted with twisting beam suspension the fact that the beam has to twist to allow independant rear wheel movement means it acts as an antirolbar, manufacturers would specify the thickness of the U section to achieve the stiffness they required. so any additional bar can be avoided purely by stiffening up the beam. usually by the simple process of converting the U section into a box section by welding plates across the open side. however at best this is an inexact way of stiffening it and as far as i know was only used as way round competition regs that prohibited additional of non standard antiroll bars but did allow for the standard suspension components to be strengthened.
going way off topic but following in the spirit of "strengthening" suspension components, a certain engine tuner, best known for his work on mini's, campaigned another make of car in the british touring car championship in the '70s and was repeatedly crticised by his competitors etc for retaining the standard fuel tank location under the boot floor which mean he had to carry extra weight to protect it, rather than follow convention and mount it in the boot. what no one realised at the time was that his fuel tank protective structure had the effect of forming a smooth surface from the rear axle to the lip of the lower rear valence which gave him a lot less drag than the others who had a gap before the rear valence then presented the airflow with a flat vertical surface