So we've gone down a bit of a path here, and thats cool :-)
Thank you for taking the time to respond and ask. My original 418 message was very much intended as a light hearted joke, in the spirit of "if we wanted to return cheeky responses to previously nonsense APIs that AI invented" I actually like this idea of subverting AI in inventive ways.
Now to the point where we've got here, yes I 100% agree in real-world, production applications, you should return response codes which accurately represent the actual response. But there also a place for fun, even in production, and 418 represents that for me.
Thanks for caring about quality :-)