What are these metal plates covering parts of PCBs called?
That is a shield and is used for many purposes.  It can be to protect a sensitive area from EMI/RFI, or it can be to protect the rest of the board from EMI/RFI generated in that area or to help pass emissions testing.

If it is soldered down it can be problematic to remove as the large area acts as a heat sink preventing the solder form reflowing.  Using a low temp heat removal system (using bismuth based solder) would work.

NOTE: this is not a "shield" as per Arduino usage - which is why the Arduino usage is a poor selection of terminology.