From the current specification and also noted on Wikipedia on
https://tools.ietf.org/html/rfc8259:
8.1. Character Encoding
JSON text exchanged between systems that are not part of a closed
ecosystem MUST be encoded using UTF-8 [RFC3629].
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.
Implementations MUST NOT add a byte order mark (U+FEFF) to the
beginning of a networked-transmitted JSON text. In the interests of
interoperability, implementations that parse JSON texts MAY ignore
the presence of a byte order mark rather than treating it as an
error.
From
https://www.json.org/ :
string -> \ -> u -> 4 hex digits
UTF-8 allows for decoding of 4 byte characters.