The FAA used to require systems like PBA on the Sikorsky 76....but no longer do.
The PBA was for a specific requirement about longitudinal stability and stick force gradient - I don't think the requirement to demonstrate compliance, has gone, it just depends on the type and how it is achieved, whether naturally or artificially.
Technology moves forward.....Certification Authorities always lag behind technological advances
This is hardly a question about technology. We are talking about a 50+ years old APU that was designed to run for short periods, and fairly steady demand, whilst on the ground, with a much lower requirement for component failure probability. It is now being used with variable power demand, over long periods, in widely changing atmospheric conditions and experiencing dynamic loads, such as gyroscopic precession (64,000 RPM turbine!), it MAY not have been demonstrated or designed to meet.