Prolonged thumb sucking can influence the development of the teeth, upper jaw, and surrounding facial structures, particularly when the habit continues during the years when the permanent teeth and facial bones are developing. The changes most commonly discussed are an increased overjet, an anterior open bite, and changes in the position of the upper incisors and maxilla. However, a less prominent Cupid’s bow or a flatter/less defined philtrum is not a classic or well-established direct consequence of thumb sucking. The appearance of the Cupid’s bow and philtrum is influenced by genetics, lip anatomy, the position of the upper teeth and jaw, facial growth, and changes in soft-tissue volume. So, while prolonged thumb sucking could potentially influence the surrounding anatomy indirectly, I wouldn't attribute a subtle change in the Cupid’s bow or philtrum to thumb sucking alone. If the habit occurred during childhood and stopped years ago, it is also unlikely that the sucking motion itself is continuing to reshape the lips now. The more established long-term effects are dental and jaw-related rather than specifically flattening the Cupid’s bow.