I'm not sure if there will be a macOS version soon without the AVX2 requirement. Based on what the Mac team says here, I would guess that the BG3 running natively on a macOS requires AVX2 because the game is being built using the latest Metal API (for the GPU).

But removing the AVX2 requirement might be possible later, with optimization.

I know this is obvious, but the Windows version of BG3 is built with different libraries (apparently not requiring AVX2), and that is why the game can run on Windows on a non-AVX2 Mac machine.