The mask data was moved to Terrain_Tile which is a tile of the terrain map in base class of Level hierarchy. It was just much better that way than storing mask data to Display_Map, because it started to get weird with access to Display_Map instance from Level class. Now the data is pretty much where is belongs. It takes more memory, but that's not an issue, because levels spend quite small amount of memory. The data is reset when the save game is loaded, but it's also not a problem, because the mask data is useful only when creating the level.
Today I could try to finish all the routines which actually use mask data. Sometimes using mask is a lot easier than finding out particular terrain tiles, but sometimes you need to find specific type of tiles to create some stuff on them. That's why both systems need to work together. The mask is set using default values for terrain and later when creating rooms etc. it's written over with new values. I find one of the hardest parts of level generation is the way you decide what can be created on a single tile, with often complex rules and special cases.