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I am not aware of any sample code, which is really a shame. Without sample code, writing USB serial device drivers is a very obscure science. However, if you can influence the code on the USB device you are better off implementing the USB CDC ACM protocol. That way your device appears under /dev/tty* and /dev/cu* without the need to write a device driver. And best of all, it works on Windows and Linux as well - again without device drivers. And on macOS, you probably don't want to use the /dev/tty* device but rather the /dev/cu* device. /dev/tty* is from the old days when you had a modem connected to your serial port and your software would become active if there was an incoming call.

I agree that there are many open question and a shortage of information and examples. And USBSerialDriverKit has disappeared without further information (in beta 3 or 4).To the best of my understanding, the new drivers / system extensions are shipped and installed with an application but they are active even when the application is not running and remain on the system until the application is removed. So the drivers are available to other applications and they are probably registered globally as well. That's a clear advantage over user-mode USB interface.