You can't turn it "off" you can only choose to see the effect or not, but the work is done by the system to compute the parallax effect regardless.
I was told this by an iOS Developer who said that the parallax is always computed in the background, and it explains why wallpapers take so long to apply even on newer devices, as the parallax is being computed and applied to your image so that if you choose to "Reduce Motion" it can be reenabled at will with no delay.
So, that's why I leave it on. The device is already doing the work for parallax, you can only choose to see the effect or not and instantly enable/disable it, but in the end if the device has done the work and is using resources regardless, I may as well keep it enabled.
If you could truly turn it "off" there would be a substantial delay in turning it on/off ... you are merely choosing to see or not to see what the device has already pre-computed.