There are two different concepts mixed here. USB CDC class, and UART.
Such thing as "The UART" does not exist. UART is a thing that can be implemented by anyone in any way they want, so UARTs from different eras of computing and from different vendors have different properties.
Technically it is of course possible, nothing prevents using 16 data bits in an asynchronous start stop frame. In practice, it is unlikely that any commonly used UART has ever implemented 16 data bits, as that would be quite an exotic special case.
1 start bit, 9 data bits, one optional parity and two stop bits is approximately what typical UART implementations provide.
Now, what parameters the USB specification allows for a virtual communication interface is another thing. It may just take all combinatios you could have into account, even if no such thing exists.
You should remember, that the CDC exists to communicate with many types of devices, and the fact that there is an actual UART is a special case. Most CDC devices don't even have an UART. Sometimes they may have an USB UART chip, to communicate UART with the MCU, but all that is unnecessary if the MCU has an USB interface, so there is no UART anywhere in the data path.
So the CDC is a superset of many communication interfaces. The CDC might have parameters to support a communication interface other than UART which does have 16 bit symbols. Or, it might be there to allow 9 bit UART frames like MCUs, as everything up to 8 bit frames fit into a byte.