Ah ok, fair enough. This is a more recent (2017) clarification of the standard which I hadn't seen. The original mid 2000s specification did not require UTF-8.
> Previous specifications of JSON have not required the use of UTF-8 when transmitting JSON text. However, the vast majority of JSON-based software implementations have chosen to use the UTF-8 encoding, to the extent that it is the only encoding that achieves interoperability.
Hmm, not as I read it. It says that UTF-8 is the 'default' encoding. In context that just means that it's the encoding you assume if the first four octets don't match a pattern characteristic of one of the other encodings (when restricted to ASCII characters). See section 3 of https://datatracker.ietf.org/doc/html/rfc4627. The original RFC is vague, but I think the idea is that a fully conformant implementation would support all the encodings mentioned.
> Previous specifications of JSON have not required the use of UTF-8 when transmitting JSON text. However, the vast majority of JSON-based software implementations have chosen to use the UTF-8 encoding, to the extent that it is the only encoding that achieves interoperability.