A bit of quick research talking to several other users tends to indicate that this feature was dropped when Generation 4 products were introduced.
Hopefully, we'll see some comments from Generation 4 owners to confirm on this.
FWIW. I have both a 4th Gen AirPort Extreme and Time Capsule. Neither have the Robustness option in the AirPort Utility either. My guess would coninside with Bob's comments in that this option was most likely "dropped" by Apple after the 3rd Gen model. I would further guess that this may have something to do with an updated antenna design in the newest models not requiring or supporting this option.