Is there any reason why operators leave a space at the bottom of the boarded doors?

Solution 1:

I doubt that there is a lore reason, but the small gap beneath the door can be tactically advantageous to defenders.

In order to place a breaching charge on the door, attackers must walk up to the door and stand still as they attach the charge. A defender can watch for feet in that little gap and fire their weapon through the barricade once the attacker stands still, destroying the placed charge and possibly killing the attacker.

If the gap at the base of the door was not there, attackers would be free to move around and setup their attack route without the defenders being able to know where they are.