Not uncommon for spur to be exposed on RC's.
As you'll notice it is slightly recessed relative to the chassis plate, even if so by only a millimetre or two.
As such, any bottoming out generally does not exert any impact to the gear.
Now, running in thick brush or something may induce the likelihood of something getting snagged up, but doubtful.
Think of it like trying to throw something through the spokes on a bicycle wheel when it's spinning quickly, even though there is lots of space for an object to go through, the rotational speed of the spokes create a 'barrier'. The spur is spinning so quickly, that anything that does make it up in there is repelled back out. Even if something does make its way up in there, it is displaced inside the chassis, no harm no foul.
Wouldn't worry about it...and to answer your question it is not common practice to encapsulate the spur gear bottom chassis side. Is there a risk of damage...sure, but nothing you should be consistently worried about.
Just run the truck and enjoy brother!
