Never heard about the abandoned planning documents for the first UFO. There were plenty for Apocalypse, though. Where can I read about those?
I'm not aware of any place that provides that information. I've never seen abandoned documents for UFO, but I am assuming they exist on the basis that almost every software project has extensive goals when it starts, and ends up removing some of those goals during the course of development, in order to meet deadlines and other targets (install size, game pacing, game interface complexity). In the case here, the printed strategy guide that I remember refers to things you'll never see in the game, which is a common indicator that the product design team meant for those to be in the game, and briefed the guide author on it, but then the programmers omitted it from the final game - usually because they didn't have time to finish that feature and do all the other more important things that needed to be done by release day. Sometimes, the guide author will have had access to a pre-release build where those features were partially working, but they got removed from the released game because they were too confusing, or detracted too much from the core game, or were too buggy to fix in the time remaining. (Guide authors may forgive even fairly nasty bugs if you warn them "Don't do X or the game will crash." Paying customers will expect that either you can do X safely, or the game won't let you try it. So, if fixing the crash is hard, the developers might decide to just block doing X at all.)