"Secret" recipes should be fine, as you point out, if they it can just be marked as "secret" yet still contain flags (i.e. contains: gluten, nut, dairy, etc.).

"Secret" recipes should be fine, as you point out, if they it can just be marked as "secret" yet still contain flags (i.e. contains: gluten, nut, dairy, etc.). I don't see this is too much of a problem, especially for higher-end restaurants because most of what you're paying for in said locations is the experience of the cooks and the quality of ingredients.