A lot of stuff is casually forgotten. It may live on in a private collection or a dusty filing cabinet somewhere but not accessibly.

People used to design scientific calculators out of a relatively modest number of discrete transistors and diodes. Telephone central offices used to run on a hardwired relay computer called the "marker". How many people still walk this earth who can design stuff like that? More immediately, with your phone casually using integrated circuits with transistor counts in the billions to watch a Youtube video... people used to watch off-the-air colour TV where the TV in question was made out of less than two dozen vacuum tubes, equivalent to 1-2 transistors each. That stuff is still easily found but in another few decades will also be essentially forgotten.

Oh, and what would it take to manufacture, say, Cibachrome paper and Kodachrome film? I know the Polaroid instant film technology has been laboriously resurrected but lots of obsolete chemical processes are essentially lost.