I don't know the US situation but in most of the world copyright on unpublished materials is held by whoever owns the literary estate. Once it's published it will expire in 70 years. This shows how sick it can get:

https://www.theguardian.com/books/2000/jul/22/poetry.books

The claimed holder of the rights in that case is an American though the materials are in the UK.

Mark Twain successfully embargoed his autobiography for 100 years after his death.