Currently you cannot auto create linework to chase a shaded area of a surface that represents the slope that you are wanting to highlight - you would have to turn on Surface Vertex Snap and then draw a linestring around the triangles that are shaded in your model to achieve that. What I guess you could do is to export the surface as a TTM or DXF 3D Faces and then re-import it and during import bring in the Triangles as 3D Face Linework (Linestrings. You could then delete the lines that are outside the area / inside the area of colored triangles of your surface, however I am not sure that would be faster than drawing the boundary by hand using the Surface Vertex Snapping. Depending on why you need the linework specifically the following was a trick we did a while back around this
In this instance a customer was mapping terrain for customers and the deliverable was a color coded map that showed the different slope areas of the surface. The problem of course is that TBC only colors the triangles for one defined slope range and they had several. So what we did was copied or ceated multiple surfaces from the same data (one for each slope range), and then set the slope range color mapping of triangles to the different range for each surface. Each surface was given a unique surface color, and we shaded the slope areas in that surface color. This gives you a composite color map that shows the entire surface and the different color areas show the different slope areas for the entire model.
While we did not create linework, we created the deliverable in this manner and the customer was pretty happy with the result. If you start with Points and Lines you can create x surfaces from the same source data and then just change the properties for each surface model to generate the composite color map.
I think there were two applications - one was Farming (water drainage) and the other was Wind farm placement
Sorry I cannot give you a quick answer to the specific question you asked, but hope that this helped and provided some ideas.
Here is an example where the colors represent (surface is not cleaned or anything and has silly triangles in the edges but it serves to how the point I guess
0-10% Slope (Red)
10 to 20% Slope (Dark Green)
20 to 30% Slope (Purple)
30 to 40% Slope (Cyan)
40 to 50% Slope (Bright Green)
>50% Slope (Grey)
Appreciate that this is an OG Survey model surface but it would be tricky for the software to find the Top and Toe of each slope range from this anyhow, it would be likely easier to draw out those lines by hand using the slope arrows display for the surfaces to show the Top vs the Bottom or to use the 3D View and draw the lines in in that view. I guess if this was a design surface then the slopes are likely more uniform etc. so maybe easier to chase out.
Hope this is useful
Alan