While doing some research on the net, I encountered that there is a (beta) driver
396.18 available for Fermi GPUs on Linux, which is even supposed to support
For reasons unknown to me, this driver seems to have disappeared from Arch, AUR, and, consequently, from Manjaro as well. There was a discussion about that driver in the past at [Testing Update] 2018-04-30 - Kernels, Trizen, Compiz - Testing Updates - Manjaro Linux Forum
Is there a reason why nobody is using that driver and instead, everybody is preferring the (apparently older) driver
390.xx for Fermi GPUs?