arnold does not have a toon shader.. but one is in development... https://arnold-rendering.com/2017/11/02/toon-shader-test/ you could try to work with the facing ratio... https://support.solidangle.com/display/A5AFMUG/Toon+Shading+with+the+Facing+Ratio+shader
Hum , but in maya 2018 there isn't any other render system than arnold if i'm not wrong ? So all the maya toon effects are useless ? Sorry really not used to maya rendering