[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.