In email's case this is true, but in JSON's case it's not. JSON is a format defined as a sequence of code points; MIME is defined as a sequence of octets.
What is the 'bytes support' issue for json? Is it about content within a json text? Or about the transport format of a json text?
The question is whether the json parsing should take bytes or str as input, and whether the json marshalling should produce bytes or str. More specifically, the question is whether it is ok to drop bytes.
I personally think that it needs to support bytes, and that perhaps str support is optional (as you could always explicitly encode the str as UTF-8 before passing it to the JSON parser, if you somehow managed to get a str of JSON to parse).
However, I really think that this question cannot be answered by reading the RFC. It should be answered by verifying how people use the json library in 2.x.
The standard does not specify any correspondence between representations and domain objects
And that is not the issue at all; nobody is debating what output the parsing should produce.