One can take a 2D shape and convert it into a 3D object by extrusion over a short distance into a Z-axis (using the Context Menu, Covert > To 3D for example). This is great; but - why can't we convert back? This has two levels of difficult: 1. Shapes which were originally 2D: For those, one can keep the original 2D data and just restore it - discarding the extrusion info and the 3D orientation. This should be rather easy to implement, although it might require storing the shape information differently, so potentially an ODF change (or maybe not, I can't say). 2. Any shape: Here, one could offer to project the shape onto the plane of the canvas, flattening it. The color or other aspects of the faces and edges could be made to depend on the angle, distance etc. - or it could be set more arbitrarily.
I think, this report covers different requests and should be split. for 2.: The request is to convert what you actually see into a 2D vector graphic. Correct? Currently the complete rendering is converted to a bitmap, when you use 'Convert' or SVG export. It would be possible to make a group of polygons, each polygon filed with a bitmap. You need a bitmap fill because the 3D multi light effects are not represent-able by a gradient fill. Such vector graphic could be used in SVG export too. But who will spent money or time on an implementation? for 1: For extrusion, you can use custom-shapes. Such have the toggle-extrusion mechanism. Currently there is no UI to convert an arbitrary polygon to a custom-shape or extract such from a custom-shape or to edit the polygon. But that is only a missing UI and not a problem of file format or of our internal model. It is possible to get back the creating polygon for the case of 'To 3D' and 'To 3D Rotation Object' tools. That polygon is stored in the shape. That is essentially an UI problem. Albeit it makes only sense if the 3D-Scene contains only 1 of such objects. For arbitrary 3D-scenes, there is no way other then method 2, to get a 2D object. For example the 'cube' shape has no generating polygon; it is neither an extrusion nor a rotation object.