Alan Tiegreen, best known for his illustrations for Beverly Clearly’s Ramona Quimby books, is still with us at somewhere between 89 and 90. (Here’s hoping he does as well as Cleary herself, who made it to 104.)
According to her Wikipedia entry, this is what happened. Duncan was on the sitcom Funny Face. After the first episode of that show, Duncan had surgery to remove a brain tumor behind her left optic nerve. She is no longer able to see using her left eye. Duncan and her doctors decided to leave it in place. It still moves with the right eye. She does not have a glass eye.
I found this line in his wikipedia article unintentionally funny:
In British film, along with Larry Dann, Angela Douglas, Patricia Franklin, Hugh Futcher, Alexandra Dane, Jill Goldston, Valerie Leon, Jacki Piper, Anita Harris, Bill Cornelius, Marian Collins, Valerie Shute and others, he is now among the last surviving actors to star in multiple Carry On films.