I don't normally like making posts like this but...
My theory is that they've been quietly retired. They are almost redundant due to high amount of complex content we now get.
Forum Discussion
I always assumed that the game as originally fashioned included the idea of “levels” because that was simply the way games of this sort typically work. My gut feeling is that if the TSTO designers could magically go back and do it all over again they would abandon the level concept. It’s an unnecessary distraction.
