Back in 1998, Congress passed a law adding 20 years to the term of all U.S. copyrights. That means that, for the last 20 years, no copyrights have expired, and no new material has entered the public domain.
That all ended on January 1, 2019, when all the copyrights from 1923 will expire and the works they protected will be freely available for anyone to use. The most prominent example of is Robert Frost’s “Stopping by Woods on a Snowy Evening,” which is now in the public domain.
Copyrights from 1924 will expire on January 1, 2020, 1925 copyrights will expire in 2021, and so on and so forth.
Things will get very interesting when the copyright to Mickey Mouse expires on January 1, 2024. To learn more, please see this article from Smithsonian — For the First Time in More Than 20 Years, Copyrighted Works Will Enter the Public Domain.
Many thanks to fellow BAIPA members Wendy Bartlett and Ruth Schwartz for bringing this to our attention.