In newly generated worlds, there is a bedrock, at the usual y=0 rather than the new y=-64. 0.
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This was a conscious decision from the Mojang team to split the update in two, you can read more about it in this article.
1.17 has some of the content (mostly blocks, items and mobs) while the next update (1.18) will have the world generation you saw in Minecraft Live, the deeper caves, and the taller mountains.
Hope this helps!
Duplicate of MC-223144, which was later resolved "Works As Intended".