Complex sentence: Some tall structures and skyscrapers have been proposed that have not yet been built, and may never be, in some cases because construction hasn't begun, or it did but was never completed, and the plans have subsequently been abandoned.
Split into simpler sentences: Some tall structures and skyscrapers have been proposed that have not yet been built, and may never be. Some structures have been proposed, but construction was never begun, or if begun never completed, and the plans have subsequently been abandoned.