Does solid/multi-stranded/Litz wire make a difference in binary serial output?
For all practical purposes, there is no difference between stranded and solid wire.  The only exception is for very high speed signals (>1 GHz), where solid is arguably better.

The main purpose of stranded is to make the wire more flexible, and has nothing to do with AC resistance at high frequencies.

There is something called the Skin Effect, where high frequency signals tend to travel on the surface of the wire.  But stranded wire behaves as a solid conductor in this regard, because the electrons can travel freely from one strand to another.  Litz Wire, on the other hand, has insulation between the individual strands and so does behave better with high frequency signals (how much better is open for argument).  I have never seen a commercial product use Litz Wire (ignoring audiophile people).  I'm sure there is a practical use for Litz wire, but suffice it to say that it is rare.