As it turns out, because they're really swell folks, some of the Bethesda developers working on Skyrim did make a concious decision to unhook and hide away content that had to be cut from the game, but not entirely delete it from the game's files, with the hopes that modders would be able to uncover it down the line.
As with any big, ambitious game, there was plenty of stuff that didn't quite make it into the realse version of Skyrim and has since been ressurected by modders, who aren't under the same stresses to get a full game finished and shipped in as playable a state as possible as Skyrim's developers would have been once development reached a certain point. The arena in Windhelm is one example that springs to mind straight away, but I'm sure there are others I'm drawing a blank on.
Well, interestingly enough, some of the files that let modders know things like that were ever even supposed to be thing were sneakily left in the game by the devs. This comes from former Bethesda developer Joel Burgess, who worked on Skyrim in addition to a number of Fallout games and Oblivion, during an interview at the C3 modding showcase.