CO2 lasers tend to be very high power. Often they will have 100 W in the beam. They can be much higher. Some will burn through safety goggles about as fast as you can blink.
Optical elements used with such lasers need to be designed for such power. When light strikes a coating, it is transmitted, reflected, or absorbed. Absorption must be kept down to a few tenths of a percent to avoid overheating. Reflectivity must also be low. Even so, light reflected from an antireflection coating on a concave surface can come to a focus and start a fire.
So I expect there isn't much demand.
That said, if you have a need, companies that manufacture such coatings can make custom coatings. There is no reason that a notch filter cannot be made. Look for companies that make lenses and other optical components for CO2 lasers.
Custom coatings tend to be expensive. They are sputtered in a vacuum chamber. For standard coating, the chamber can be filled with optics. For a custom coating the entire run is for your part.