Copyright Aid

Although there is plenty of supposedly "public domain" material on the internet, I notice that in the vast majority of cases, they stick in what I would call a "get out clause" passing the responsibility onto the prospective user. Here is an example, and it makes quite interesting reading, it is from the Library of Congress, USA, but they are themselves publishing the images online, so how safe is it really to use such material!?

https://www.loc.gov/rr/print/195_copr.html