Did Mandarin Pinyin `chiang` used to exist?
The 反切 in 廣韻 use Middle Chinese pronunciation, so one cannot take the modern readings of the 反切 上下字 as a guide to how a word might be pronounced.

In this case, you are looking at words in the 江 rhyme category. These words have developed a -j- medial in Mandarin (when there is a velar initial) that was not present in Middle Chinese. Other dialects have not. Compare, for instance, the Cantonese pronunciation of 江 "gong".

Although  is not a "living" character, words which are in the same initial and rhyme categories are still around, such as 撞 zhuang. This is the regular development. 

Mandarin has never had retroflex initials followed by medial -j-; in fact, this would seem to be a very unlikely combination in any language.