I’ve been researching since I 've tried Arducopter 3.4.rc1 and has some problems with the flight mode Loitier . It seems that this version has been replaced by NKF ( neural kalman filter ) .
Someone has more information
It seems that no one else has noticed the change of filter.
3.4rc1 is using EKF2, I think that in the context of Ardupilot NKF should be read as Nonlinear Kalman Filter, not Neural.