The serial:// port was initially implemented by Carl Sassenrath to use with
some INSTEON home automation devices. Presentation here:
https://www.youtube.com/watch?v=Axus6jF6YOQ
The source samples were integrated to make a functional cross-platform implementation by Joshua Shireman. The implementation was split between a "core" portion that implemented the abstract port (picking out Rebol settings like baud rate or parity from the port spec object), and a per-platform "serial device".
Joshua has periodically brought the implementation up to date and demonstrated it to work on some old serial devices from his closet. :-) However, the Ren-C developers do not have any such devices, and the extension has no active users. Circa 2019, the WebAssembly Build is higher priority (which excises the device model of R3-Alpha entirely). So there is not much attention paid to testing this code.
However, it is kept compiling--in large part because of any thinking points that the code might bring up about API usage in extensions. However, that means that a "sufficiently motivated individual" probably wouldn't be far from having a working serial interface, if they wanted one! It gives a pretty good idea of what POSIX and Windows APIs one would use, and how to use the API to extract settings from Rebol objects into C datatypes.