Based on the description that emphasizes benefits for digestion, lowering cholesterol, cleansing the intestines and weight control, this is most likely a food high in soluble fiber.

A typical, simple and historically widely used food that meets all of these requirements is:

Oatmeal (or other oatmeal or whole grain cereal)
Why is oatmeal so beneficial?
Lowers cholesterol: Oatmeal contains a large amount of soluble fiber called beta-glucan. This fiber forms a gel in the digestive tract that binds bile acids containing cholesterol and helps eliminate them from the body, thereby reducing the level of “bad” LDL cholesterol.

Digestion and bowel cleansing: The high content of fiber in general (both soluble and insoluble) promotes regular bowel movements, cleanses the intestines and acts as a prebiotic (food for beneficial intestinal bacteria).
Weight control: Fiber causes a rapid feeling of satiety, stabilizes blood sugar levels and thus helps to naturally control food intake.
Other possible “healthy food of grandparents” with similar effects
If it were not oatmeal, other candidates from traditional cuisine with a high fiber content and probiotics are:

Legumes (lentils, peas, beans): An excellent source of fiber and vegetable protein, which are proven to help lower cholesterol and improve digestion.
Sauerkraut / Fermented vegetables (Pickles): It is rich in probiotics, which are extremely beneficial for the intestinal microflora, as well as fiber.
Kefir / Sour milk: Traditional fermented dairy products that are also full of probiotics and were consumed daily in the past.