I tried to get frame duration by calling ope_encoder_ctl, but this call ended up with an errorCode=OPE_UNIMPLEMENTED (-15). But as far as I can see this operation was long ago implemented in libopus. Why it is not available in libopusenc?

Code example:

  opus_int32 frameSize;
  int errorCode = ope_encoder_ctl(encoder_, OPUS_GET_EXPERT_FRAME_DURATION(&frameSize));

Libs versions:
libopus - v1.3.1
libopusenc - master (commit 6e0a98c)