One reason for taking it out is that the code holding it together was quite flimsy, even by the standards of some of the stuff in NeoLemmix. It already caused a lot of problems to continue making it work when Cloners were added. Another reason is that improved level selection altogether is on more more-long-term todo list, and at this point, the cheat key would become irrelevant anyway.
However, if there really is a lot of interest, I can put it back in the next update.
I will note that even using the cheat key, you still need two presses - the cheat key, then a click (or press Enter) to move from the results screen to the desired level's preview screen. So, it's only a difference of one key press, although it does involve extra fade in/out time.