Is it limited to cable ties and elastic bands, or would plastic mounting brackets be allowed as a non-functional decoration?
Your question is answered by rule <R7f>:
Thus, you can use pretty much whatever you want to manage cables on your robot.
Just to clarify, this rule only covers cable management items that are commercially available.
I don’t think 3D-printed cable management items would be allowed. (I remember there was a Q&A about it, and I seem to remember the answer was that they weren’t allowed.)
Good point. Though in theory there’s nothing stopping a team from designing some cool new 3D printed cable management doodad, then selling copies on EBay or whatever other site so that it’s “commercially available” and thus legal for use. Probably would be a good fundraiser if the doodad proved especially useful.