[Question] The tech left behind

Hello, I am ask­ing for some in­sights for a re­search I am do­ing. Can you cite ex­am­ples of tech­nolo­gies that have been for­got­ten? What I mean by “for­got­ten” is not things we don’t know how to do but used to (I sus­pect there aren’t that many), nor things that are no longer in use but used to (me­chan­i­cal tele­vi­sion), but things that were de­cently de­vel­oped (ei­ther in the­ory or in prac­tice) but never “saw the light of day” any­way.

It’s my first time post­ing, so I won’t do much polic­ing on the an­swers, thanks in ad­vance.