That could be. I heard the same thing as you did, Janie, from music teachers and choir people during 60s and 70s. I did and still do find it much more likely I'll have thick, hard-to-rid mucous if I'm short of sleep or dehydrated than from milk or other dairy. My mother ensured I drank milk at every meal as a child and also taught me to drink water in many small sips if I had a frog in my throat of any origin. Through the years I've found that when I do have throat grunge to clear, as long as I'm well hydrated it's gone by the time vocal warm-ups are done, 30-60 minutes. Maybe being faithful with hydration and warming up is my key to successfully ignoring the no-milk or no-dairy imperative. But I'm largely wondering aloud, as I can't say one way or another with any objective certainty. The human body has so many variants, as does environment and the choice of things we each consume.

Interesting mix of experiences posted.