This is the Frei0r.sigmoidaltransfer filter from Janne Liljeblad.
Desaturates image and creates a particular look that could be called Stamp, Newspaper or Photocopy.
Brightness: Controls Brightness of image. Range 0 to 1
Sharpness: Controls sharpness of transfer Range 0 to 1
Both parameters default to 0 in Kdenlive - which is unfortunate because this results in a totally black frame. You need to have numbers above zero to see the effect.
This effect appears to have arrived with version 0.9.5 of Kdenlive