Actually, I can for two reasons. First is of course the RFC mentions that items can be registered after the fact, if it's found that a particular well-known suffix is being widely used. But the second is a bit more chaotic - website owners are under no obligation to consult a registry, much like port registrations; in many cases they won't even know it exists and may think of it as a place that should reflect their mental model.
It can make things awkward and difficult though, that is true, but that comes with the free text nature of the well-known space. That's made evident in the Github issue linked, a large group of very smart people didn't know that there was a registry for it.
https://github.com/AnswerDotAI/llms-txt/issues/2#issuecommen...
Excuse me???
""" A well-known URI is a URI [RFC3986] whose path component begins with the characters "/.well-known/", and whose scheme is "HTTP", "HTTPS", or another scheme that has explicitly been specified to use well- known URIs.
Applications that wish to mint new well-known URIs MUST register them, following the procedures in Section 5.1. """
Having said that, this won't work for llms.txt, since in the next version of the proposal they'll be allowed at any level of the path, not only the root.