This is to make hitboxes completely clean (no red and blue lines) without losing the ability to see entities like players or items through blocks. Let me explain:

It isn't hacks because you can see the entities through walls like this in vanilla. The blue line from any hitbox is what can go through blocks.
So if a player is facing towards a wall you can see them if you are on the other side IF you have hitboxes on because of their blue line that goes through the wall.

I will certainly look into adding this as a new feature, Thank you.

Much appreciated!