If you want to know when changes to NWS products are coming, monitor this page:
https://www.weather.gov/notification/From that page, you can request to subscribe to a mailing list to get messages when new public info statements & service change notices are posted. NWS has a formal change management process, including requests for comments on upcoming changes.
In most cases, new links/output/etc. are available ahead of the changeover date to facilitate software updates.
I'm going to guess that the XML/XSLT based UI got very little to no traffic and it wasn't worth the time to update the tech. The public mostly gets alerts from mobile emergency alerts (WEA/EAS), the news media, and weather apps and those outlets don't need to direct people to a copy of the same data decorated with the best of what 2002 Web design had to offer.
There's also that XML for Web presentation purposes was abandoned with the release of HTML 5.