What are these parts on STOL/bush planes?
Those are not specifically STOL hardware, they are bungee covers that happen to have an airfoil shape.

Aircraft like the one in your picture use bungees (also called "shock cords") to absorb the force of landing: As weight is transferred to the gear the bungees are stretched. The installed bungees look like this:


The bungees are literally just "aviation-grade" bungee cords, and would be quickly deteriorated by weather if left exposed, so they are usually covered. The most basic covering is a leather or plastic boot:


The boots are not much more aerodynamic than the exposed bungees however, so some aircraft (including many with STOL kits) have a cover with an aerodynamically favorable shape to reduce the drag caused by the bungee bulge. That style of cover is what's shown in your picture.
Here's a closer look: