Slicing is a lot more important than some keyword. And design-wise, filling the numpy namespace with singletons for keyword to other things in that same namespace just makes no sense to me.
At least from the perspective of discoverability, you could argue that string constants form a namespace of their won, and so growing the “string” namespace is not inherently better than growing any other.
I don't really think those are valid arguments. Strings are not a namespace, and if you want to make the analogy then it's at best a namespace within the functions/methods that the strings apply to. Discoverability: what is valid input for a keyword is discovered pretty much exclusively by reading the docstring or html doc page of the function, and not the docstring of some random object in the numpy namespace.
Ralf