The question is certainly of historical interest but of little current relevance. It’s troublesome to design for anything but standard USB class - CDC. Even if say FTDI may not be offering a chip for that, all you need is a small cheap microcontroller with built-in USB peripheral to act as a bridge.
In practice, a Teensy will do, either as a complete module, or just the MCU programmed with code that uses Teensy[duino] core libraries. Making a serial-to-USB bridge that way involves a paragraph or two of code to set things up and that’s it. Teensy libraries work with more chip variants than just the exact ones on the Teensy boards. There are of course other open source libraries that work across many chips, like, say, tinyusb. I just found Teensyduino libs to be a “no-brainer” of sorts.
There are Arduino boards out there that use a separate tiny microcontroller as a serial-USB bridge - it's cheaper and less cumbersome for everyone than dealing with USB-to-serial bridges. A CDC class is nowadays driver-less and works on just about everything but iOS (sad face).