Quite independent of whether namespace axis be deprecated or not, the xpath itself is not clear and that's the problem. I guess you want to find what kind of uri associated with a certain prefix? The question itself is not very generic to xml recommendation as the prefix is something allowed at the caprice of those authoring the document.
But, if you really want to take a look of it, you can still do something like this.
I would not be too enthusiatic with that, but it serves the intention of knowing what can be done and what cannot.