This was such sad news for me. I was fortunate enough to see her in The Apple Tree at least three times for the entire show, and at least four or five times for Acts 2 and 3. Since I was 15 at the time, I much preferred Acts 2 and 3 to Act 1, and took advantage of lax intermission controls at the Shubert to sneak in for those comic acts. Eventually, I realized that, without Act 1, the show would not have existed.
Fifty-two years later, I still believe that her performance in The Apple Tree was arguably the greatest musical performance I have ever seen (IMO, her competition for the top slot would be Angela Lansbury for Mame and Gypsy, Glenn Close for Sunset Blvd, and Nathan Lane in The Producers (Patti Lupone would probably be on the list for Anything Goes, had the role of Reno Sweeney not been given such short shrift in Act 2).
The prior season, I saw her in On A Clear Day, and fell in love immediately.
Those two performances provide an important place in my personal theatre history. I was devastated that she stopped performing live, at least in NYC. I have always wondered what shows were not produced because her unique talents were not available, or what roles were played by people not as talented as her. For example, (not to dis Dorothy Loudon), I think she would have been a great Miss Hannigan; I think she would have been a great Norma Desmond, a great Dolly Levi, a great Desiree Armfeldt, a great witch in Into the Woods, a great Reno Sweeney, and etc. Based on some of her excellent film performances -- her performance in Nashville was wonderful low comedy, and the ending of the movie with her singing "It Don't Worry Me' remains one of the great movie endings of all time to me -- you could add a lot of non-musicals to the list as well.
i have never understood why she gave it all up, but I hope it was for good personal reasons, since it deprived several generations of theatergoers of seeing one of the greats.