This discussion from the IRSA is about battens and not, strictly speaking, sail thickness. Not to disagree with John's engineering analysis it is worth pointing out that the term "soft sail" is defined in the ERS issued by World Sailing as:-
A sail where the body of the sail is capable of being folded flat in any direction without damaging any ply other than by creasing.
The discussion is laid out on the M Class website so suffice it to say that there appear to be 3 options available with regard to the current batten rules for M and A class yachts.
1) Do nothing and see what happens, but this could lead to further requests for interpretation and/or protests at a major event.
2) Change the rule somehow to prevent "soft sail" material being used as a batten and bypassing the rules. A potential minefield, especially for measurers.
3) Take away the rule altogether
This discussion does not apply to the 10R class as they have no batten restrictions, and where no abnormal sails have appeared as a result.