I think you're misreading "into his 90s" as "the 90s"? ;)

OP says "released since the 90's and beyond," earlier in the comment.

No - JRR Tolkien was already dead by 1980.

I read it in 1982 when I briefly lived in London as a child.

thanks! you are correct, I thought Unfinished Tales was released in the 90's when I first read it. it was released in 1980.