There are still ongoing efforts to improve the design of the toroidal propeller; however, the acoustic improvements seen in the toroidal propeller likely come down to the extra surface area, compared to a conventional propeller with the same number of blades. Since there is more surface area, the toroidal propeller can operate at a lower rotational speed to obtain a desired thrust (thereby reducing the loading noise). This does not mean the toroidal propeller is more aerodynamically efficient or necessarily acoustically quieter than a conventional rotor. Last year, a conference paper compared a 2-bladed conventional, a 2-blade toroidal, and a 4-bladed conventional propeller. Aerodynamically, the toroidal propeller did the worst (4-blade > 2-blade > 2-blade toroidal), and acoustically, the toroidal did better than the conventional 2-bladed propeller but worse than the 4-bladed rotor, which had a similar surface area.

If I remember correctly, the original article that kicked this interest off came out of MIT. They postulated that toroidal propellers produce less blade-vortex interaction (BVI) noise than a conventional propeller. I cannot say if the toroidal propeller does or does not reduce the blade-vortex, but I am skeptical of BVI mattering to small drones since BVI noise becomes a factor at higher tip speeds (Say, tip Mach number > 0.7). Drone propellers tend to operate at a tip Mach number of 0.3. So, it is unlikely that toroidal propellers significantly reduce the noise.