I was surprised by this warning when I was opened a stereo project.
Turns out an effect was using binaural panning. This means when using headphones, the listener will hear a simulated surround sound (after all you only have two ears for real surround sound too).
Interestingly even with the setting disabled in Preferences, I can still hear the effect with headphones as I drag around the puck…
I’m leaving it disabled since I don’t need all the actual surround features listed in Logic Pro X: Surround features overview. Hopefully that’ll also make Logic run faster.