The next major update to Minecraft is a biggun, so big that it's now getting split in two. Mojang have announced that due to technical complexity and number of changes they have planned for the Caves & Cliffs update, it will now roll out in summer and holiday releases. The 1.7 update was previously expected this summer, which will now be focused on adding new blocks, items, and creatures. The winter release will include the big world generation changes that add new cave types, mountain types, and increasing the world build height.
In their announcement post, Mojang call out world generation as one of the big sticking points for the update. "Take the completely revamped world generation, which affects all the caves and mountains in Minecraft," they say. "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."
In Mojang's video up there, they explain that they didn't want to delay the entire 1.17 update just because world generation needed a bit more time in the oven. Instead, they're splitting off the bits that can be added to the game with less headache and getting those into an official release this summer. You will be able to update your existing worlds to the versions released this summer and this winter, Mojang say.
This summer Mojang will add new Minecraft mobs like goats and axolotls and glow squids to the game alongside biome blocks. They say that most of the world generation changes will be in the winter update, though adding blocks like copper to world generation will come this summer. Good new for Nate, who recokons copper is the best new block after trying it out in a Caves & Cliffs snapshot.
New cave and mountain generation, along with placement of biomes, will be pushed off to the winter update. The archeology features, where you're able to unearth artefacts at dig sites, is getting bumped even further down the road, unfortunately.
If you want to keep testing out features for the winter release in Minecraft's beta branch "snapshots" they say you can do so. They've turned off winter release features in snapshots by default but you can choose to enable them. They explain how to do that over here.
Alongside the technical concerns, Mojang also mention that the Covid-19 pandemic has continued to affect their workflow, another factor that led to the big Caves & Cliffs split.
Mojang haven't mentioned a specific date for the first part of the Caves & Cliffs udpate, still coming sometime this summer.