Wearing a shirt is not necessary for boxing, at any stage. If you mean Amateur Boxing, according to this page it says you have to wear it.[1]. Head gear and regular groin protection are used as well. "Competitors wear protective headgear and gloves with a white strip across the knuckle. A punch is considered a scoring punch only when the boxers connect with the white portion of the gloves. Each punch that lands on the head or torso is awarded a point. A referee monitors the fight to ensure that competitors use only legal blows"