Categories that have failed RFDO

If MewBot doesn't create them, it doesn't stop someone else from doing so anyway. So the list would probably need to be built into the category tree modules somehow, so that the module can automatically tag or trigger an error when they get created.

The obvious, simple, low-tech solution is to make a page that links to all of the categories that have failed RFD, a bit like WT:RFVA, and periodically check for blue links.

That works too. But harder to maintain.