The Caves & Cliffs update is now going to be two separate updates.
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Mojang was planning to release a major update for Minecraft during the Summer season. It was a big Caves & Cliffs update that was planned that would make significant changes and additions to the game. Mojang has called it their "most ambitious" update for the game that they've ever developed. Sadly, that singular update has now been split asunder and will instead be released in two parts. One will still arrive this Summer while the second half will be held off until the holiday season.

Mojang says that there are "significant technical challenges" that the team needs to overcome before parts of the update are ready to be shipped to the public. Given the areas that the update was going to tackle, namely the caves and mountain biomes, some significant changes have to be worked on as far as the world generation is concerned. And well, that's no easy task apparently. Some issues could "wreak havoc on your meticulously crafted worlds" if things weren't perfect before release.

Take the completely revamped world generation, which affects all the caves and mountains in Minecraft. With this feature, we are completely reshaping the Overworld, including the build height. Not only do we have to consider every effect this can have on the world but also how it will affect performance. Basically, having more stuff in the game also slows it down and we want to avoid that.
Beyond that, Mojang says that they are taking into consideration the health of their workers. They say that if they stuck to their plan to release everything in the Summer, the entire team would have to work long hours. They also factor in the issues with working from home during the pandemic. Any change to development a studio can make to avoid crunch is a good change.

Mojang came up just short of detailing what content will be split off. You can get a sense of what to expect from the update, in total, over at the Minecraft Wiki page.