I came upon this quote on p.51 of National Geographic's Sublime Nature: Photographs That Awe and Inspire.
The earliest citation is from "Clare Boothe Luce", playwrighter who became one of the first female U.S. Ambassadors. In 1931 she published a novel titled “Stuffed Shirts” which contained the following passage.
Yes,” continued Mrs. Gunn, patting Lucile’s hand condescendingly. “I have resolved to grow old, naturally and gracefully, content in the knowledge that the greatest intellects are the homeliest ones, and that the height of sophistication is simplicity.
The most famous is from 1977, when the Apple Computer Company created a brochure for its new product the Apple II personal computer stating:
Simplicity is the ultimate sophistication
while then in 2000 a multi-page advertisement from a liquor company appeared in a magazine, stating
the advice of Leonardo da Vinci who wrote: “Simplicity is the ultimate sophistication”
“Not every quote you read on the Internet is true.” – Abraham Lincoln