I probably don't disagree on the API for xarray or similar library. But that choice would be up to the library, not Python developers. Whatever the way keyword argument are passed to .__getitem__()[*], they are available for the class to do whatever it likes with them.

[*] In my opinion, passing the keyword argument using anything other than the standard **kws style sounds crazy. We have perfectly good ways to do that for every other function or method. Don't invent something new and different that only works with .__getitem__().