Not only is there evidence in image or text form, there’s also audio. The ones that are the most interesting to me are the ones made for the campaigns, as they show how scenarios and stories were originally planned to be before being cut.
I made a few videos compiling them into a digestible format. It took me multiple iterations, but I managed to arrange them in a way that provided context within the campaign progression.
There’s probably plenty of unused voicelines for the AI personalities meant for Skirmish gameplay, but I haven’t played around with them enough to know which ones to set aside.