The problem isn't that Node supports this error code. The problem is that it's used by the registry.

Heck, technically the problem isn't even that it's used by the registry, the problem is the npm client is a broken untestable mess and users seem to routinely unearth extremely surprising bugs.

This all has very little to do with teapots. The fact they used 418 and didn't provide a useful error message only makes it more interesting to talk about.