I'm having trouble finding a source with a search engine, but I've heard of bronze age shields that were impossible to reproduce with current metallurgy. Ancien

I'm having trouble finding a source with a search engine, but I've heard of bronze age shields that were impossible to reproduce with current metallurgy. Ancient concrete recipes are rediscovered occasionally. These aren't fields though. The loss probably wasn't as simple as older technology being surplanted by newer tech. I think what you are looking for my be found in farming though. I would think there are entire avenues of research abandoned.