In 3.6, we introduced a new prefix, 'f', so there was no back compatibility issue. There was, however, a combinatorial explosion issue, as 'F' was also added (a mistake, I now think), and no order requirement (possibly another mistake). Hence
stringprefix ::= "r" | "u" | "R" | "U"
grew to
stringprefix ::= "r" | "u" | "R" | "U" | "f" | "F" | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"
New unordered 'd' and 'D' prefixes, for 'dedent', applied to multiline strings only, would multiply the number of alternatives by about 5 and would require another rewrite of all code (Python or not) that parses Python code (such as in syntax colorizers).
I think you're exaggerating the difficulty somewhat. Multiplying the number of alternatives by 5 is not the same thing as increasing the complexity of code to parse it by 5. A new string prefix, 'd' say, would seem to be the best way of meeting the OP's requirement. That said, I'm about +0 on such a proposal, given that there are already reasonable ways of doing it. Regards Rob Cliffe