BUT. If 60 degrees of bank is a limit, how do we legally teach the 'Max Rate' turn at around 65 to 70 degree bank angle? An exercise which I feel offers scope for advanced handling appreciation to suitably aware student pilots.
The FAA has been using the old Chapter 49 definitions. A 90 degree pitch down (or up or roll), split "S", loop, and barrel roll as helicopter aerobatic guidance in the absence of any flight manual maneuver limitations. A bank greater than 60 degrees would not be considered aerobatic by the FAA. Nor would a quick stop with a pitch-up of 60 degrees.
There has been a tendency by the FAA to use a 'reasonability' criteria. If something is considered a reasonable helicopter operation, like a quick stop or flare in autorotation, or max rate turn about a point; then it is an intentional maneuver required for 'normal' flight that does not require 'abrupt' changes in aircraft attitude.
As an experienced display pilot you know when a something is an 'intentional maneuver involving an abrupt change in an aircraft's attitude, or abnormal acceleration, not necessary for normal flight' and what isn't. Manuevers like loops, rolls, in-plane veritical 360 degree pedal turns can be performed without abrupt changes in attitudes, but are not necessary for normal flight. A quick stop may be necessary but when performed with a too abrupt change in aircraft attitude it borders on the aerobatic. One can quibble with exact rates and attitudes but we all know when a maneuver is taken to an extreme and when it is considered normal.
Though I do not have knowledge of every pilot action taken by the FAA for aerobatic flight in helicopters, I very much doubt that any pilot would ever be violated, or an aircraft grounded for inspection, because of a max rate turn at up to 90 degrees of bank as long as there was no maneuver limitation or restriction prohibiting the maneuver in the Rotorcraft manual.
Backward Blade
I am aware of the circumstances with which the Red Bull pilot and aircraft are certificated for public display in the US; and can state with certainty that Red Bull, the FAA and the International Council of Airshows worked cooperatively to mitigate risk and protect the public in accordance with all existing regulations. Listing everything the Red Bull team was required to do, and the things the Red Bull team decided they should do would be difficult here as the lists are very exhaustive; but to my knowledge the entire operation is being conducted with the upmost professionalism.