I think this is true for most specs that are sufficiently old and sufficiently complex: there will be some parts that were specified, but never widely used, some parts that were once used, but are not relevant anymore, etc. etc. For an onlooker, these curiosities might seem fascinating. If you are setting out to implement the spec and don't have an infinite amount of time however, the multitude of corner cases, some important, some not (sometimes without an obvious way of distinguishing between the two) will get on your nerves pretty quickly...