I understand that axial-flow turbine engines generally include stator vanes behind every(?) compressor and turbine stage. This makes quite intuitively sense, because without them the rotating blades would mostly swirl the air around through the engine, which would not only prevent the stages from properly transporting/compressing/expanding the air in axial direction, but also put useless rotational kinetic energy into that air. The latter is presumably also the reason why turbofans have such guide vanes behind the fan, too.
All of this applies just as well to unducted fans/propellers. Yet I've never seen stator blades behind an open propeller. There are contra-rotating coaxial propellers of course, the idea of which seems to be similar: get rid of the useless solenoidal air motion. But actively contra-rotating both props adds a lot of complexity, and seems somewhat unnecessary if the goal is mostly to remove large part of the swirling motion. Cancelling all torque is not necessary for a plane.
Especially, I would think stators could also be shaped to help retrieve some energy from the tip vortices like a duct does, without however interfering with the inflow that seems to be the reason why ducts make props inefficient at low speed.