But it does mean that there's no way to encode with Opus while targetting a specific quality level rather than a specific bitrate. And "targetting a specific bitrate" is nothing other than constraining the bitrate to some degree.https://wiki.xiph.org/OpusFAQ#How_is_the_bitrate_setting_used_in_VBR_mode.3F
How is the bitrate setting used in VBR mode?
Variable bitrate (VBR) mode allows the bitrate to automatically vary over time based on the audio being encoded, in order to achieve a consistent quality
The bitrate setting controls the desired quality, on a scale that is calibrated to closely approximate the average bitrate that would be obtained over a large and diverse collection of audio. The actual bitrate of any particular audio stream may be higher or lower than this average.
Hence, it's quality based.