Minecraft 21w43a
21w43a is out and adds blending of old worlds.
These writeups assume you have read the Official Changelog before continuing.
Table of Contents
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.