In older versions of TC, a planar 3D polyline, as you get if you section a 3D object for example, could be selected and exploded down to 3D polyline segments, then while still selected, converted into a 2D polyline. If the polyline was on a plane other than World plane, or not parallel with World plane, the resulting 2D polyline would be an ortho projection of the polyline onto World plane, but if the polyline was oriented parallel with World before join polyline, the 2D projection is true and could be reoriented back to its original position.
TC doesn't have that capacity anymore: Explode a polyline and invoke join polyline and the selection set is dropped, try and select a 3D polyline segment to be converted to 2D and TC refuses to recognise it, auto join just won't join the segments together unless you've invoked that it should be a 3D polyline.
One consequence of this is that convert to curve, useful for smoothing polyline sections through noisy scan meshes, can't be used on sections in newer TC versions without a lot more hoop-jumping than the old.
The newer mode of operation isn't just inferior because it doesn't give visual indication of the join having taken place, it's inferior because the newer mode is less capable than the old. Bring back the old, or have the option for either.