I just want to know if a backplate would protect the circuit board of the graphics card from dust or not.
Nope, it absolutely does not matter. Dust will get sucked in from the GPU fan regardless if there was a back plate or not. The purpose of the GPU back plate is to prevent sagging that bends circuit board from flexing not dust.
In fact with a back plate it may accumulate more dust between back side of the circuit board between the back plate.
I just want to know if a backplate would protect the circuit board of the graphics card from dust or not.
Nope, it absolutely does not matter. Dust will get sucked in from the GPU fan regardless if there was a back plate or not. The purpose of the GPU back plate is to prevent sagging that bends circuit board from flexing not dust.
In fact with a back plate it may accumulate more dust between back side of the circuit board between the back plate.