According to the documentation you can set surface placement mode for both feature layers and graphics overlays.
For GraphicsOverlay I can find a SceneProperties property exposing LayerSceneProperties (for setting the placement SurfacePlacement enum).
However, I can't find a similar property to set the placement mode for a future layer (such as ArcGISSceneLayer).
Am I missing something or it cannot be done after all?
This property only applies to client side rendered geometries, meaning GraphicsOverlay and FeatureLayer, to help drive how it should render these 2D geometries in 3D space. This doesn't apply to a layer like ArcGISSceneLayer which only contains objects that are already made specifically for 3D.