Not sure if this influences your thinking at all, but whatever duration copyright applies for starts once it was made, whether it was published or not.

> Copyright in a work created on or after January 1, 1978, subsists from its creation ... [1]

And "creation" basically means "written down" [2]. IANAL.

[1] https://www.law.cornell.edu/uscode/text/17/302 [2] https://www.law.cornell.edu/wex/fixed_in_a_tangible_medium_o...