If we want space tech that was here and then disappeared: manned missions to the moon. And after retiring the Space Shuttle, the US even lost the ability to put

If we want space tech that was here and then disappeared: manned missions to the moon. And after retiring the Space Shuttle, the US even lost the ability to put people into orbit until SpaceX came along. Of course that's all really about money and not knowledge, but that's true for all recently disappeared tech: it just wasn't economically viable. For tech that has actually been forgotten, you have to go back to Roman self-repairing underwater concrete.