Everyone who was working on any commercial codebase knows that this functionality are either features which nobody knows they are there, but when they disappear

Everyone who was working on any commercial codebase knows that this functionality are either features which nobody knows they are there, but when they disappear all the hell will break loose, because there is non-trivial amount of people using them and other thing on seemingly "dead code" is covering of edge cases for bugs which developer is unable to trigger, but end user will trigger them with 100% success rate.