It seems a working patch is ready for kernel 5.9: https://aur.archlinux.org/pkgbase/nvidia-340xx/?O=0&PP=10
- Bug#972430: nvidia-legacy-340xx-driver: Fails to build w... jim_p
- Bug#972430: Fails to build with kernel 5.9 jim_p
- Bug#972430: nvidia-legacy-340xx-driver: Fails to bu... jim_p
- Bug#972430: nvidia-legacy-340xx-driver: Fails to bu... KeyofBlueS
- Bug#972430: nvidia-legacy-340xx-driver: Fails to bu... jim_p
- Bug#972430: nvidia-legacy-340xx-driver: Fails to bu... Williams, John
- Bug#972430: nvidia-legacy-340xx-driver: Fails to bu... jim_p
- Bug#972430: nvidia-legacy-340xx-driver: Fails to bu... jim_p
- Bug#972430: nvidia-legacy-340xx-driver 340.108-6~bp... Mathieu Malaterre