https://hackaday.com/2018/11/30/new-part-day-the-twenty-five...
Looks like someone already made (at least the source) for an adapter based around that MCU family available:
https://github.com/zhuhuijia0001/ch559-usb-host
As for relative lack of availability, I think that says more about how many people want to use an old keyboard with a new computer (semi-common) vs. a new keyboard with an old computer (rare).
