After the latest update of Solus (I updated yesterday) the NVidia driver 390.x seems to be missing. The hardware drivers program says there are no hardware drivers available and the software manager also no longer shows the NVidia 390.x driver.
Was this driver removed intentionally or did I just discover a bug?
Removed intentionally. Nvidia deprecated the branch like 6 months ago. So it no longer gets patches to support newer kernels / x11 updates etc.
Harvey OK, that means that those who had installed this driver reverted automatically to the Nouveau driver?
So, if we want to install an Nvidia driver in Solus, must we take the Nvidia driver 470.x?
penny-farthing OK, that means that those who had installed this driver reverted automatically to the Nouveau driver?
Correct.
penny-farthing So, if we want to install an Nvidia driver in Solus, must we take the Nvidia driver 470.x?
No. It all depends on what your nvidia card supports. "Hardware Drivers" (Doflicky) in your system menu will recommend a driver for you. If it does not, it is because there are none that actively support your card in which case the nouveau driver is all you have and are already using.
Harvey In fact, my computer is equipped with an Nvidia GeForce GTX 1050 Ti card.
So, the reinstalling of an Nvidia driver via the "Hardware Drivers" menu has installed the following packages:
. nvidia-470-glx-driver-modaliases
. nvidia-glx-driver-32bit
. nvidia-glx-driver-common
. nvidia-glx-driver-current
. nvidia-glx-driver-modaliases
penny-farthing
Anything ending in -modaliases is for hardware detection purposes, so it knows what will work for you.
Main thing is that it is installing nvidia-glx-driver-current and its related packages which means your card supports the latest main nvidia driver branch which is currently at 530.41.03. This makes sense as I have a GTX 1060 (Which is from the Pascal series like your card) and can use the same driver branch.
Harvey Thank for your feed-back and these useful information. 
Harvey as a deprecated 390 user I have to ask: would you recommend installing an Nvidia legacy 390 driver from the Nvidia website? They offer them. https://www.nvidia.com/Download/driverResults.aspx/196213/en-us/
Also, it appears Nvidia is supporting the 390 in that link above. I know I am missing something here, so forgive me if I got it wrong.
brent would you recommend installing an Nvidia legacy 390 driver from the Nvidia website
No. You will get breakages installing drivers that way. The driver as is, is broken, see below.
brent it appears Nvidia is supporting the 390 in that link above
"Supporting" it in the sense they haven't deleted the driver from their website. From memory they deprecated the driver branch 1st January 2023. So it will get no more updates. It does not work on kernels 6.2 or greater without being patched. Staudey already gave it a brief reprieve and patched it once but needed more work on 6.3, as a general rule when nvidia kills support for it, so do we.
EDIT: Confirmation of upstream status for 390
The Linux 390.* legacy driver series is the last to support GF1xx ("Fermi") GPUs. Support for new Linux kernels and X servers, as well as fixes for critical bugs, will be included in 390.* legacy releases through the end of 2022.
Source: https://nvidia.custhelp.com/app/answers/detail/a_id/3142
