Um...send a proper request? This seems like a perfectly fine code to me. Suppose you have an HTTP request that takes two parameters and the client only sends one of them. Which of the other existing 4xx codes would be appropriate for that situation? "400 Bad Request" is probably what you'd use currently, but as defined it indicates a syntactic error with the request rather than a semantic one.
Presumably the client either does or does not know how to send a proper request so if it is receiving the error message it might as well behave the same way as any other 4xx code.
Indeed. But, again, we could already do that without the 428 code. And so the situation is: response code 428 tells the client nothing beyond the info gotten from any 4xx response code. The response body tells a human something. The addition of the new response code helps neither one.
Um...send a proper request? This seems like a perfectly fine code to me. Suppose you have an HTTP request that takes two parameters and the client only sends one of them. Which of the other existing 4xx codes would be appropriate for that situation? "400 Bad Request" is probably what you'd use currently, but as defined it indicates a syntactic error with the request rather than a semantic one.