4. Unknowns Information / opensource / perfect documentation scarcity?

Something that is definitely no longer protected by patents but the original inventor / copycats haven't documented how to build it step by step and/ or insufficiently (possibly even for themselves?) and it is inherently complex?

e.g. Fogbank: Lost Knowledge Regained https://cdn.lanl.gov/files/nwj2-09_62c62.pdf