With the announcement that the Caves & Cliffs update is being split into two smaller updates, the Summer Update and the Holiday Update, all of the features moved into the Holiday Update like world generation changes and the new world height have been removed from the current snapshot. That said, there is a way to patch the changes back in, it’s pretty easy to do.
1. Download The Caves & Cliffs Prototype Data Pack
2. Create a new world, and click on Data Packs
3. Once on the Data Pack screen, drag and drop the .zip file you just downloaded onto it.
4. When it asks if you want to add the pack, hit yes.
5. The added pack will appear in Available. Move it into Selected and hit Done.
6. Once you create the world, it will be an experimental world, and ask if you want to proceed. Hit proceed to open your brand-new Minecraft Snapshot world.
This will give you a new world with all the Holiday Update features that have been released so far intact! This is world-by-world so that you can have patched and non-patched Snapshot worlds in the same directory. Repeat this process to add more patched worlds. It will ask you to backup and/or proceed or not open every time you access the world. Choose the option that works best for you.