Negative Review of Restaurant-Loshon Horah or Not?

Yes.

Go to page 92 in the Artscroll book: “Chofetz Chaim A Lesson A Day”

the first paragraph says: “Making a statement about a product that would discourage people from buying it constitutes Loshon Hora as it could harm the livelihood of the manufacturer or the storekeeper who sells it.”

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my commentary (I couldn’t find the source in the book upon a quick look through)

if asked by an individual about particular establishment, you may be able to tell them about your experience, however, your motives need to be pure (not revenge for the bad service or meal) and you cannot embelish your critique (the length of the wait for the service or delivery of the food, the neatness or lack of neatness in the dining area).