How to remove the height of Cesium World Terrain
Are you asking for a pure ellipsoid without terrain deformations, like the default globe in CesiumJS? We don’t support ellipsoid terrain unfortunately, and it’s not on our current roadmap.
Can I just modify the unity code to achieve terrain without height without modifying the source code?
No, probably not. You can supply your own ellipsoidal globe via ion or local file, if you have one.
You could maybe fake a globe using a regular Unity sphere, and scale it based on the WGS84 ellipsoid’s dimensions. But you won’t be able to use raster overlays on it to drape imagery.