From the glossary:
Though it can be used today, Its practice has been banned, and all knowledge of its implementation has been/will be stricken from the Ludus. Many of the necessary elements have been made illegal/extinct, and its use is immediately detectable by the Custodi.
The purpose of its use was typically to achieve that which could not be achieved by the technology of the time, in order to “bend the rules” without detection. The tools and machines would disintegrate into carbon and absorb back into the ecosystem, leaving no trace.
One curious item that contradicts that is the Voynich Manuscript, which is believed to be a Legiuminascea-based medical book. It is perhaps because this book is written in a “dead” language (or code in which all keys have been destroyed) that it is allowed to exist.