berarma I think I've read Camera2D as a child of KinematicBody2D is the way to make the camera follow the player. Where does it say it shouldn't be used that way?
It doesn't say that anywhere. I just have a lot of experience and that is what I discovered. Maybe for simple tests you can make the camera a child, but in a real game you need to customize the properties of the camera (such as the bounding box, so it doesn't go past the edge of the screen) which you can't do properly if it's a child on the object it's supposed to be following.