This is 100% correct. All you need to make a driver/kext with a valid signature that will permit it to be loaded without warning and without disabling SIP (system integrity protection) is an Apple Developer account. And not a special enterprise one or anything - anyone willing to pay the $100/year needed for a Developer Certificate can sign their own kexts and release them for macOS.
There is no way Apple could be ‘blocking the drivers’, it isn’t like kexts can only be installed through the app store. In fact, kexts are NEVER handled by the App Store.
There is no legal (via EULA violation etc.) or technical way Apple could block any third party from releasing software or drivers for macOS, and beyond that, what reason would Apple have to do so? None. The last thing they want is to reduce the already limited pool of hardware that will work with macOS.
But don’t take my word for it… take Apple’s.
I think a much more likely explanation is that, unlike previous macOS releases, Mojave moved away from OpenGL entirely and requires the GPU fully support Metal 2 for even basic GUI acceleration. This would require a significant rewrite of earlier drivers, and that is generally easier/simpler to do the older a GPU is, simply because there is a smaller feature set to support and simpler architectures.
Regardless, I think we all already have the answer as to what is going on simply by looking at what Nvidia cards DO have Mojave Drivers: Kepler series. This is probably the oldest cards Nvidia wanted to support at all, and they are also the “easiest” (I mean that in a relative sense - I’m sure it is anything but easy) cards to do a complete driver rewrite for.
And that is why those are the only ones working. Simply put, Nvidia hasn’t finished the drivers for newer cards yet, and certainly wasn’t going to have them finished in time for the Mojave release.
We could at least in part blame Apple for dropping OpenGL, but honestly, this is mostly on Nvidia for not devoting enough developers to the macOS drivers. I’m sure the people who ARE working on it are doing their best and making progress, but given the lack of any ETA, there is probably still a good deal of work to be done, so we can either try and get NVIDIA to assign/hire more people to work on it (unlikely they will just because we complain though) or be patient.
I’ve got a GTX 1080 and need CUDA, so I’m very much stuck on High Sierra, so I’m getting bit as hard as anyone else by this - but as much as I wish it weren’t so, having patience is probably all any of us can do for now.