What about mother in apple cider vinegar?
There has been multiple debates about this. The Pecanbread site has it as illegal for concerns that is contains yeast. While there is no mention of it in the official SCD website or book. Supposedly some SCDers did some research a while back and found that there is no yeast in it, just bacteria cultures. However there is no evidence to support either readily available to confirm one or the other. You will just have to make your own judgement on it. Like all things with microbial cultures you might be fine with it while others might have reactions.