I recently participated in a local (small) gravel event that had some single track. After the ride, a few riders complained about its inclusion in the ride (“too technical”, “doesn’t belong”, “need more warning”, “should’ve given an option to avoid”…and my favorite sarcastic response: “great XC course with some roads in between”).
Riders seemed to come with the right “gravel” equipment (bigger tires, cyclocross/gravel frames/bars/drivetrains, etc), but many were ill-prepared and quite put-off by the narrow, rocky single track.
I gather in other parts of the US (and perhaps elsewhere) this isn’t as popular? But here in the northeastern US (PA, NJ, DE, etc), it seems to be very common to repurpose mountain bike courses to capitalize on the increase in popularity of this type of riding.