Not all of the voids were proper rooms; some (like room 5, if I remember correctly) were hardwired into the game to perform certain actions; others are possibly caused by hooks for game sections only partially finished, or at least not finished in the revision of the data files to which I have access.
Nonetheless, there are some omissions which I greatly regret not having available; probably most notable of these is the UBIK maze by "White the Creator" (Jim Finnis) which was mostly written whilst said wizard was in a surreal state of drunkenness, visiting Campbell House terminal room, UCL, London.