Dick,
Rubbish, ICAO has not stated that transponders cannot be used as a risk mitigator in safety studies in relation to airspace.
Wow ... we're really getting into semantics now....
You CANNOT [yet], however, design a system that relies on aircraft-to-aircraft surveillance or separation based on the use of transponders – that is, the use of airborne collision avoidance systems, like TCAS and Mode A/C transponders, as an airspace design tool, is prohibited.
..... Effectively, ICAO – and the ANC specifically – has determined that ACAS MAY NOT be used as a risk mitigator under any circumstances in the design of airspace management procedures. ALL ICAO Technical Panels have been directed that they may not factor such systems in the development of separation standards, the design of procedures, or the development of future systems. ACAS is a “last line of defence” for pilots – NOT a system design tool.
ICAO ANNEX 11:
SECTION 2.4: DETERMINATION OF THE NEED FOR AIR TRAFFIC SERVICES
2.4.1 The need for the provision of air traffic services shall be determined by consideration of the following:
a. the types of air traffic involved;
b. the density of air traffic;
c. the meteorological conditions;
d. such other factors as may be relevant.
Note:- due to the number of elements involved, it has not been possible to develop specific data to determine the need for air traffic services in a given area or at a given location. For example:
a. a mixture of types of aircraft of varying speed (conventional jet, etc) might necessitate the provision of air traffic services, whereas a relatively greater density of traffic where only one type of operation is involved would not;
b. meteorological conditions might have considerable effect in areas where there is a constant flow of air traffic (e.g., scheduled traffic), whereas similar or worse meteorological conditions might be relatively unimportant in an area where air traffic would be discontinued is such conditions (e.g., local VFR flights);
c. open stretches of water, mountainous, uninhabited or desert areas might necessitate the provision of air traffic services even though the frequency of operations is extremely low.
2.4.2 The carriage of airborne collision avoidance systems (ACAS) by aircraft in a given area shall not be a factor in determining the need for air traffic services in that area.
I think the "spirit" of the Law is that ... if you can't use transponders as a design element, then you surely shouldn't be using them as an implementation element ... especially in non-surveillance airspace.