Up until a little while ago that part was called the buttplate catch, but in new literature it's been renamed to the floorplate catch. FWIW, "floorplate" seems to have become a common term among manufacturers.

Reminds me of when the center rail on the bottom of the slide, which strips the top round from the magazine and pushes it forward for feeding, used to be called the "stripper rail". It stripped the top round from the magazine, after all. Nowadays it's common for it to be called the "pickup rail" (it picks up the top round). I guess somebody decided that "stripper" rail was too insensitive.

Nomenclature changes over time. Sometimes for simplification, and sometimes to keep current with social norms.

__________________
Ret LE Firearms inst & armorer