I went to drive the car, but it would not go in reverse

Unfortunately, you cannot. These metal slivers are most likely from the clutch pistons or even the drums inside the unit.

Any time you find metal in the pan then that means there is more metal in the unit that you cannot see.

So, we are going to have to remove the unit and rebuild it or replace it.

https://www.2carpros.com/articles/automatic-transmission-problems

I am attaching the process below on how to remove the transmission as this is unfortunately the next thing we are going to have to do.

Please run through this and let us know what questions you have.

Thanks