Who said that they ought to make a collection of diaries and letters after the war?

You're likely thinking of Winston Churchill, who made this suggestion in 1945.

He believed that a collection of diaries and letters from the wartime period would provide valuable historical insights. His own wartime diaries became a major source for his later historical work, and he felt others' experiences would be equally important.

While not everyone heeded his call, many individuals and institutions did preserve these vital materials, creating a rich archive of the war years.