It would be a pretty cool feature to have in Audacity. Being able to Flip The Audio/Notes, for example, this video: https://www.youtube.com/watch?v=PYQQrx-PxoQ or well it might just be a stupid feature but I like the thought of the feature being in Audacity.
I think that effect has been done by flipping the notes in a “MIDI sequencer” rather than any “audio” effect.
Audacity is an “audio” editor, with only very minimal support for MIDI, so this effect would not be possible in Audacity - you would need a MIDI Sequencer.
(There’s some info about MIDI Sequencers here: https://www.sageaudio.com/blog/recording/midi-sequencing-basics.php)
I think i have heard something similar done to an old Christmas song, but that was maybe 20 years ago…
Really sounded wry and mangled though. 
What i assume the process was, is mirroring the the spectrum on an axis in the middle, like 700Hz or so.
But i suppose that would mean splitting the spectrum in semi tones (at least), which is something like 120 bands (at least), and then pitch shift each band to the opposite side of the 700Hz axis.
I’m just a simple end user, but i suspect this would be quite a challenge to achieve as a usable Audacity effect…
Would be fun though !