My understanding from Rivian is that the standard underbody of the skateboard chassis is aluminum, carbon fiber and "alloy", but not steel.
From this I gather that it's not a matter of protection (add on the $1700 shield) or no protection at all.
We're paying $1700 for steel. For you rock crawlers, I'd say go for it. For the those of us for whom "off road" will likely and only occasionally take the form of driving on an unpaved USFS road, probably save the $1700 for something else.
A few years back I shopped a hybrid SUV and was told that if I drove on an unpaved surface it would void my warranty. "Off Roading" meant that you were parked in your driveway.
Now here is Rivian saying they built for off roading, so please go off roading, Me, I'm good with the standard underbody.
But, again, I defer to and respect you true rock crawlers. God Bless You. If $1700 buys piece of mind then you should do this.