How about a technology that was lost in the 20th Century? I've been learning more about Florida. Not the current politics, but
it's Pre-Columbian history.
In the early 1900s Rockefeller, Ford and Henry Flagler were interested in the oldest city in the USofA. St. Augustine, FL. There in 1833 Franklin W. Smith built a house of slip-formed concrete using a mixture he'd learned from the locals.
The Ponce De Leon was built in in the 1800s and other buidling in the city, but Flagler stiffed Smith is a real estate deal and he died in poverty, the people he worked with passed and th erecipe for his 'coquina mix' concrete was lost.
Those buildings still look brand new today. They will outlast all that Brutalist architecture from mid-Century.
I think this has to do with the recent discovery of the secret of Roman concrete, which is the ratio of quick lime and slaked lime. So we can build infrastructure to last thousands of years.