CageEdit The PreserveStructure option is not available for polysurfaces and is not supposed to be displayed if a polysurface is selected as the caged object. From Help CageEdit | Rhino 3-D modeling
The PreserveStructure option does not apply to polysurfaces, and will not be displayed if polysurfaces are selected for editing.
The option is displayed if a polysurface is selected. Version 7 SR12 (7.12.21299.13001, 2021-10-26)
Create a box
Select the box
PreserveStructure is displayed as an option