Broke a thin piece off this piston. Not sure what its for. Can it still be used?

not usable for me.

Nope. Teardown time, in my opinion.
I'd sure want to know the why & how, and who the piston manufacturer is.
I'd think there's debris everywhere, as well.

Piston skirts can be broken while out of the engine by letting the rod flop onto it to hard. Been there done that lesson learned.

I did worse than that. I was changing the wax ring on a toilet and when I had it off cleaning the base the oak seat feel open and smashed the tank. So off to the store to spend $200 for the original $.99 repair. Lol

Its not the skirt, its above the wrist pin. I hit it with a piece of wood trying to get the stuck piston out.

Junk.

Trying to save pistons in a stuck motor is a waste of time. Save the block and the crank, get new pistons to fit the over bore the motor will need anyway. Gene

Once the steel rings weld themselves into the cast iron cylinder walls you will need to have it bored out to get rid of those stuck parts damage to the cylinder walls from the rings to the cylinder walls