I only glanced at the code in ANIM.CPP but for a C++ program from 1997 it looks kinda nice, right? The methods are all short, with some descriptive comments at the top of each one. Inputs and outputs described. Nice index at the top of the file. I’ve seen worse!
Normal software I find is a bit different because things like UI libraries give you a lot of structure already. In game code, you do so many things custom, and performance matters so much (each frame needs to finish around e.g. the 17-millisecond mark) that it's really up to you to apply the necessary discipline