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...