Mojang says in an announcement that they
have decided to split Caves & Cliffs into two parts, as
the ambition level of this update and the current state of the world has made it
difficult to deliver on this promise while sticking to [the] original timeline.
The first part of the update will be releasing in mid-2021 while the second part will be releasing at the end of the year (
around the holiday season, respectively).
The first stage is now what is in development, focusing on
mobs, cool items, and new blocks. The second part will add the world generation changes.
The Caves & Cliffs update was first announced during Mojang's annual Minecraft Live stream last October and was originally set to fully release in mid-2021.
This change reverts the world height to 256 blocks in height (down from 384) and removes all of the new cave types. Worlds made in previous snapshot are no longer openable in the new version.
21w15a also adds block forms of the raw iron, gold and copper items added last week and adds a new data storage medium for map makers called the marker.
For a guide to the changes in snapshot 21w15a, watch the video below: