Yes, the metal under the shelf is structural. It’s how they achieved the (slightly) better rear visibility and headroom in Model 3 and why we didn’t get a hatch on mini S. Sure, you can probably remove it by installing much heavier and less capable reinforcements but, if that was the best option, Franz would’ve done it; he clearly digs hatch styling and practicality.
Car camping is pretty nice even with the shelf (heck, it makes a great place to set your screen for a movie in bed). Want more room, consider the Y.