In the latest CSC (Update 2023), the only two shields that offer that much protection are the Boarding shield and the Expandable shield (when expanded "to the size of a boarding shield" (CSC p. 140). The reference in the boarding shield description is that the bonus is from providing cover to the user(s). Cover bonuses are handled under the Hiding section of the combat rules. This does add to the Travellers' armour protection, but only when the Travellers are completely hidden behind the shield and not attacking. If the Travellers want to fight back, they would be using the Cover rules instead and inflict a DM -2 to attacks against them, forgoing the +8 protection increase.
The boarding shield usually has a "vision slit or firing port" that will allow Travellers to fight without leaving cover, but the expandable shield does not offer that option.