In so many cases, these works disappeared before their copyrights expired, for example, the *majority* of silent films are lost forever.
Then, in 2019, Jenkins and Boyle got to start cataloging the works that *were* entering the public domain, most of them from 1923 (copyright is complicated, so not everything that entered the public domain in 2019 was from that year):
https://web.law.duke.edu/cspd/publicdomainday/2019/
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