I might've heard of it but ignored and forgot it because I never encountered the need to use it. Backspace is far from either of my hands when I'm making a level, therefore using "B" for this purpose is faster in all cases. No matter where in the level you are, you usually have an empty tile nearby. If you don't, you usually don't need to place one. If you do, you usually want to erase a large space, so "B" is still faster as it lets you grab a whole group of empty tiles. No matter what, backspace sounds useless to me - maybe if it were closer to the remaining hotkeys.