I must be wrong about "SCRUB_SPEED, I think it was something the original author added but never got working. So maybe the shuttle hardware really did exist back then. I will hook up the shuttle and uncomment the code to see if I can get it to do anything.
From Audacity, https://manual.audacityteam.org/man/scrubbing_and_seeking.html
Scrubbing or Seeking is the action of moving the mouse pointer right or left so as to adjust the position, speed or direction of playback, listening to the audio at the same time - a convenient way to quickly navigate the waveform to find a particular event of interest. The term "Scrubbing" comes from the early days of the recording industry and refers to the process of physically rotating tape reels to move the tape past the play head to locate a specific point in the audio track. In Audacity - *Scrubbing* changes the audio position without skipping. - *Seeking* changes the audio position, skipping over audio to move more quickly