Why are the AI and Behavior Tree sections missing from the Documentation?

When navigating to this page:
https://docs.unrealengine.com/en-US/Gameplay/AI/index.html

It lists the topics covered in the documentation. I was able to access these only a week ago but since then they have all been removed. I was learning from those since I need to implement AI in the game I’m currently working on.

Is there an important reason these sections were removed or was this just a mistake?

It has been reworked since, here is the new version: Artificial Intelligence | Unreal Engine Documentation

I believe the link you have is outdated, you might even report that as a bug.