If you mean cold meal boxes ("sad box" is sometimes also used to refer to snack boxes), yes they are serving cold meal boxes to/from the stations where they don't have meal catering contracts (generally smaller stations or ones where there are only a few flights that are long enough to qualify for meal service). There's a list on stations in the Wiki at the top of in-flight meal service thread -- Delta In-Flight Meal/Food Service: The Definitive Thread — 2022 Edition