This feature was used by many map makers as a way to set off redstone when clicking on air. (Using /testforblock)

This was very useful for having a way to "talk" to NPCs in adventure maps or as a way to interact with blocks in a way that did not require buttons.

As it is, the Piston Extension cannot be removed without the use of commands and creates an area where blocks cannot be placed.

From my understanding. It seems as though the hitbox of this block was removed and that is why it no longer works.