Most websites use undocumented RESTful APIs that are tightly bound to their current UI. Versioning UIs means versioning their undocumented backend APIs as well

Most websites use undocumented RESTful APIs that are tightly bound to their current UI. Versioning UIs means versioning their undocumented backend APIs as well. I did see some websites provide a button to "switch back to old version", but it's usually just a transition period to reduce friction. So I think the answer would be "it iterates faster than we could version it".