If w3w needed to change a code for a particular location, say because one word in the triple became offensive, there’s no reason they couldn’t keep the old triple as a hidden value so that old references still worked.
Still, w3w is a poor solution to a problem which has many better solutions.
Plus, it fails quite a few use cases. Can you buy a paper map of your locality with all the squares listed, and, an index to help you find a square (because w3w essentially scrambles the word to location association)?