Minecraft 21w43a
21w43a is out and adds blending of old worlds. See the changelog.
Upgrading Old Worlds
All bedrock between y=0 and y=4 (hardcoded) is replaced with deepslate, regardless of whether a player placed it or not.
Item Shadowing Fix
Item shadowing has been fixed. Now the game clears the player’s inventory slot after updating the container, instead of before.