She is 81+ and so this day was to come even if she has continued to write till recently with such amazing poignancy ...all short stories but each word, each sentence weighs a ton! And cuts like a knife! A brilliant writer, no doubt and all the praise coming her way the last few years (including being called the Chekov of our generation) are well deserved.
If you have not read her stories, some of them are available for free (without subscription) at the New Yorker website.
In articles reporting this decision by Alice Munro, many refer to her1994 Paris Review interview, where she said that she was "panicked" by the idea of stopping, even for a moment:
"What happens in old age can just be a draining away of interest in some way that you don't foresee, because this happens with people who may have had a lot of interest and commitment to life. ... That it might is the danger. This may be the beast that's lurking in the closet in old age -- the loss of the feeling that things are worth doing."
There are very few interviews with Alice Munro that I have read over the years and so, I'll leave you with a more recent one from the New Yorker last year.