Correct, the built-in drivers only cover the "Kepler" architecture, as that was the last one that Apple used in a Mac. The 3rd party Nvidia drivers which support more modern GPUs are only offered up to macOS 10.13 High Sierra.
The technical background for this is that Metal and OpenGL acceleration requires the GPU driver bundle to be loaded into a process's address space. (effectively a dynamic library) With 10.14, Apple has enabled the "library-validation" codesigning flag just about everywhere, which means that only libraries signed by the same developer as the process's main executable or by Apple can be loaded into a process. Hence, 3rd party GPU drivers won't load in WindowServer for example, which makes them dead in the water.
Apple could of course implement an exception or sign Nvidia's driver, but so far it looks like they're not interested. Perhaps there will be a solution to coincide with release of the new Mac Pro which once again contains PCIe slots you conceivably might want to fill with Nvidia cards, but I wouldn't count on it.
(FWIW, my company maintains the macOS driver for one of 2(?) manufacturers of USB graphics adapter chips, so I have a pretty good idea of how the macOS graphics stack works.)