Is there a block that falls when stepped on in Minecraft?

A sand block with string on top of it will act like this, but can only be placed with commands.

yes, and no... you can use a trick using sand which i will explain in detail, but you need string over all the blocks, which is not easy to hide... however, in the 1.17 update there is a block which automatically retracts when stepped on. There is a trick that works in any version: put a layer of grass three blocks below where you want your trap. Put a two tall flower on all these grass blocks, then put sand, gravel, concrete powder or any block affected by gravity on top of these two tall flowers, and then break the bottom layer of all the flowers. Then, you can remove the grass. TAKE CARE: any block update to any of the trapped blocks will cause them to all fall down. This is also how the action mechanism works: if you put string on top of the blocks, anybody stepping on them will fall. If you already put a chest on top of the sand/gravel... before breaking the flowers, opening the chest will make the blocks fall. Hope this helped!