Minecraft wiki entry for Development_versions, associated text: Removed chunk’s Level and moved everything it contained up.
Level.Entities has moved to entities.
Level.TileEntities has moved to block_entities.
Level.TileTicks and Level.ToBeTicked have moved to block_ticks.
Level.LiquidTicks and Level.LiquidsToBeTicked have moved to fluid_ticks.
Level.Sections has moved to sections.
Level.Structures has moved to structures.
Level.Structures.Starts has moved to structures.starts.
Level.Sections[].block_states has moved to sections[].block_states.
Level.Sections[].biomes has moved to sections[].biomes.
Added yPos the minimum section y position in the chunk.
Added below_zero_retrogen containing data to support below zero generation.
Added blending_data containing data to support blending new world generation with existing chunks.