dec := json.NewDecoder(strings.NewReader(","))
fmt.Println(dec.More())
fmt.Println(dec.Token())

$ go run .
true
<nil> invalid character ',' looking for beginning of value

$ GOEXPERIMENT=jsonv2 go run .
false
<nil> invalid character ',' looking for beginning of value

The JSONv1 implementation of More reports false if there is an error filling its buffer, but it reports true if the next token is semantically invalid. JSONv2 appears to report false if the next token is invalid.