Minecraft wiki entry for Development_versions, associated text: Terrain from new chunks will now blend with existing terrain from old chunks, preventing glitched borders from occurring.
The old bedrock layer between Y=0 and Y=4 in old chunks gets replaced with deepslate.
If there is bedrock at Y=0, lower places in old chunks will be filled with deepslate.
Caves do not generate below Y=0 in old chunks but lava lakes and geodes may generate.
A new bedrock layer is placed at Y=-64.
Worlds upgraded in this snapshot have these changes permanently applied.