Apparently, we have a Silent, Subliminal plugin for the ultrasonic, single-sideband technique.
https://forum.audacityteam.org/t/silent-subliminals-solved/20100/1
Unlike many other processes and techniques, this one comes with tools to get spoken messages in and back out of modulation as a check on the quality of the work. It’s not single-ended—encrypt a message and hope for the best.
Koz