An act of congress, for one.
Also, AFAIK, federal privacy generally ends at death, as does criminal liability; so releasing government files from a federal investigation after death of the subject is generally within the realm of acceptable conduct.
It seems unlikely you lose all rights when you die or it would be chaos - imagine all the secrets people die with that affect everyone they know. An integral part of every estate plan would be incinerating records. Wills do have real power.
OTOH, there's a 2004 case, National Archives & Records Administration v. Favish[1], which establishes the surviving family's right of privacy to death scene photos, but that's technically not privacy of the deceased.
[1] https://www.justice.gov/archives/oip/blog/foia-post-2004-sup...
(It also surprises me that this passed anyway, given that both sides of the aisle seem to have people with clear reason to keep it covered up... ?)
(Also, Maxwell is specifically named, and is still alive... ?)
https://en.wikipedia.org/wiki/Epstein_Files_Transparency_Act