So, in a name authority record, the birth and death dates of a person are often added:
Gorey, Edward, 1925-2000
For living persons, a birth date is often added and left open:
Winterson, Jeanette, 1959-
When a living person whose date has been left open dies, the death date can be added. But you need a source of information to cite in the authority record, saying where you got the death date information. Hence, a post from an LC cataloger on RadCat:
I can probably add the death date, but I have to quote something as a source. I have been known to attend funerals and add death dates taken from the service leaflets.
That is dedication and that is why I love cataloging and catalogers.
(I’d just link to the post, but you have to sign in to access the archives. The post was made on Thu, 17 Apr 2008, has the subject “date of death,” so if you are a list member, you can go look. Since the RadCat archives are closed and I can’t find a list statement of policy on quoting list posts in other places, I’m leaving the author name off.)