[Turing_Machine]

https://copyright.cornell.edu/publicdomain

It's complicated.

I would guess that there is an awful lot of vintage porn out there that was published without a proper notice, registration, and/or renewal of the copyright (because people were often put in jail for producing porn back in those days, it was highly unlikely that they would register the copyright).

Modern works don't require notice or registration to be protected, but the rules were different in the past.

This is U.S. law. Laws in other countries differ considerably.