Many JSON parsers allow to store extra fields in a separate dictionary/map, and certainly the format allows for it too, so I'm not sure what makes you say that statically typed languages are at a disadvantage here
> You can of course solve the same thing with JSON, but, well, YOU have to solve it.
There is not a single well established convention across all languages/impls. The default behavior in many languages if a field is missing is to either panic, or replace it with a null pointer (which will just panic later, most likely).
It does seem like some technologies get credit for solving problems that they created.