An Anti roll bar/ anti sway can work pretty effectively on softer suspensions.
When one side is compressed through weight transfer(a hard launch with body twist) or side ways(hard cornering) That compressed side will, through the bar, pull down the opposite side. Stiffer springs will not allow as much 'body roll' to actuate the anti roll bar action, hence the softer spring rates to make it work.
Full bodied drag cars with soft rear springs and without a bar, will compress and create massive body twist actually unloading the left rear tire and over loading the right. Install and tune an anti roll bar and as that body mass comes down and twists , the bar forces the whole rear body mass the compress EVENLY and load both rear tires EVENLY.
NASCAR Cup went to using soft springs and big sway bars years ago to 'lower' the car mass(center of gravity) on the corners especially on the speedways. I had been using them on MY oval track cars for 4 years before the big boys caught on.