Yes, I use a fermented drink, water kefir, to boost the activity of the flour and water to prevent overfermentation (which I initially experienced) and spoilage. There is a naturally growing yeast in this fermented drink and it helps to develop the naturally growing yeast in the starter which is thought to be captured from the air. There is also thought to be some naturally occurring yeast on the flour that also becomes part of the starter. Many people sensitive to commercial yeast can eat sourdough breads without any reactions. Some highly sensitive people, however, may still have some trouble with the natural yeast.
Historically, natural yeast sourdough breads were consumed by humans for thousands of years until just recently, the last 100 or so, years. The yeast “predigests” the sugars and starches in the bread making it easy for humans to digest. Since the advent of commercial yeast which speeds up the rise time, there is no time for the predigestion to happen so our bodies have to work harder to digest the bread. This can lead to many other digestive and immune system issues food sensitivities.
To answer your question if a starter using ambient yeast can be considered yeast free, I think no, but it is a highly digestible form of yeast that usually does not trouble the person who eats it. I consider my breads “commercial yeast free”.
Thanks for asking! Sharon glutenfreesourdough.blogspot.com