@foone coming in way late on this one, was reading RFC 8259 in order to write a JSON parser in Lisp and section 7 on strings defines a section at the end that calls out / as a character to escape. I think most encoders don't do this because it's pretty nutter butter. Of course, same section / is in the range of unescaped characters.

Guess that goes into the "must accept on decode but don't send it that way" bucket.