There are not brackets and a curly brace shapes in keynote.
However, brackets and a curly brace shapes can be made very easily.
The way is just to overlay a white rectangle on a rounded rectangle or a callout rectangle.
How to make brackets
Brackets can be made by overlay of a white rectangle on a rounded rectangle.
In addition, round brackets can be made by the drag of a green dot of a rounded rectangle.
How to make a curly brace
A curly brace can be made by overlay of a white rectangle on a callout rectangle.
