Make 405 RECOMMENDED for mutations over GET

[martinbonnin]

The status code paragraph currently reads like this:

If the _GraphQL response_ does not contain the {data} entry then the server MUST
reply with an appropriate `4xx` or `5xx` status code:

- If the failure is due to an issue in the request itself, the appropriate `4xx`
  status code should be used:
  - If a mutation is attempted via the `GET` verb, status code `405` MUST be
    used.
  - If an unsupported HTTP method is used, status code `405` is RECOMMENDED.
  - If the `Content-Type` of the request is not supported, status code `415` is
    RECOMMENDED.
  - If none of the media types in the `Accept` header are supported and the
    `Accept` header does not include `application/json`, then status code `406`
    is RECOMMENDED.
  - If the client did not produce a request within the time that the server was
    prepared to wait, status code `408` is RECOMMENDED.
  - If the size of the URI was too large, status code `414` is RECOMMENDED (and
    the client should consider using `POST` instead).
  - If the size of the request headers (or any one header) was too large, status
    code `431` is RECOMMENDED.
  - If the size of the `POST` request body was too large, status code `413` is
    RECOMMENDED.
  - If the JSON body of the request could not be parsed, status code `400` is
    RECOMMENDED.
  - If the request is not a well-formed _GraphQL-over-HTTP request_, status code
    `422` is RECOMMENDED.
  - If the GraphQL document in the request cannot be parsed, status code `400`
    is RECOMMENDED.
  - If the request does not pass GraphQL validation, status code `422` is
    RECOMMENDED.
  - If the operation to execute cannot be unambiguously determined, status code
    `422` is RECOMMENDED.
  - If the variable values cannot be coerced to match the operation's variable
    definitions, status code `422` is RECOMMENDED.
  - If the client is not permitted to issue the GraphQL request, then the server
    SHOULD reply with `401`, `403` or similar appropriate status code.
  - If the server cannot process the request due to being a short and stout
    ceramic vessel, status code `418` is RECOMMENDED.
- When the server is the reason for failure, the appropriate `5xx` status code
  should be used. For example, if the server is not able to execute requests at
  this time due to maintenance or load-shedding, then status code `503` is
  RECOMMENDED.

I love this paragraph because it gives concrete guidance what error status code to use in 16 real life cases. For 15 of those cases, it gives a RECOMMENDATION, but for the very first case, it gives a REQUIREMENT.

I find this adds unnecessary cognitive load to the spec. I would like to point people at this paragraph and say: "look, this is our recommendation but you can use something else in the 4xx or 5xx range if you want to".

The 405 exception makes everything more complicated because now I have to explain that in that specific case you can't use anything else. And I'm not sure how to explain this.

Question: is there a specific reason we disallow people to use 4xx for mutation over GET but allow it for everything else?