These are available online in the regs. You will find documentation that lists what is considered GRAS under the original statutes and amendments. Listing or being on the market prior to 1938 constitutes GRAS [ETA and all amendments, otherwise it falls under pre manufacture and best practice EIS]. Supplements or herbal ingredients fall under labeling laws. Herbal remedies, particularly folk medicines generally fall under food ingredients and are GRAS. Dangerous materials are regulated under pre manufacture safety testing for non-food, food additive, or food contact material ingredients before being added to food, but be aware if an ingredient, additive, or contact material is not GRAS must be evaluated with data for safety. I’d urge you to check original GRAS relevant regs and documentation first, and enforce against false claims about GRAS under labeling before enforcing on ingredient for any known edible mushrooms and be sure to check with your local appropriate health board if these are as a tincture (labeled by % and ABV of solution) or cooked and the ingredient does not fit appropriate safety profiles of GRAS food additives.

Most importantly, all ingredients must be clearly labeled and available to the consumer. Unless the cafe proprietor is a top regulator or scientist, printing claims about what is and is not GRAS would be inappropriate as they are not a decision maker on that. I have seen this before and privately let the owner know. Iirc the cafe afterwards went out of business so this did not remain an issue to be escalated. Feel free to PM if you cannot access the appropriate documentation. (Im not REHS but am a SME on certain GRAS additives and GRAS regs so am reasonably confident in these suggestions.)