Currently, responses without a specified charset encoding in the header are stored internally with binary/ascii 8bit encoding. According to the spec, the specified charset should be redundant. Should the default for json responses by utf-8 instead?

From RFC8259:

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.

Also in RFC8259, at the end of Section 11 (or just above of Section 12):

Note: No "charset" parameter is defined for this registration.
Adding one really has no effect on compliant recipients.