[kragen]

Uranium glass. Radium-dial watches. Gas-lantern mantles of urania and thoria. Lindane. In California, denatured alcohol. The meat of the heath hen or the baiji dolphin. Opium. Caribbean monk seal oil. Spermaceti. Ivory billiard balls. Low-radioactivity steel. Most patent medicines. Old-growth wood of some kinds of tree. Chemistry kits. The Revigator. Large sheets of monocrystalline mica.

It's probably going to be more difficult to find something that is still widely used and is manufactured, but cannot be manufactured in a particular city in 1918 but not 2020. It would require deep knowledge of the industries of a particular city or of a particular industry. For example, right now I think all three of the ruling engines in the US capable of cutting a research-grade diffraction grating into glass are in the single Richardson Gratings lab in Rochester, New York, but one of them was built at MIT before 1918. So perhaps Boston (or Cambridge at any rate) was capable of producing such artifacts in 1918, but not today; but, if that is true, rigorously establishing the truth of that claim would require considerable investigation.

Similarly, many US cities have many fewer watchmakers, compounding pharmacists, and piano tuners than they had in 1918; in some cases the number is indeed zero.