"Shoes are required to eat in the cafeteria” is an old joke sign that supposedly appeared in a college cafeteria in 1975. Underneath the sign was scribbled, “Socks can eat any place they want.” The joke was popularized in a 1975 issue of The Reader’s Digest and has been included in many humor collections.


Google Books
The Reader’s Digest
Volume 106
1975
Pg. 42:
A notice in a University of Colorado campus cafeteria stated: “Shoes are required to eat in the cafeteria.”

Someone wrote below: “Socks can eat wherever they want.”
— Contributed by Mike Shrout

20 May 1975, Bennington (VT) Banner, pg. 4, col. 7:
Humor
Cafeteria rules
A notice in a University of Colorado campus cafeteria stated: “Shoes are required to eat in the cafeteria.”

Someone wrote below: “Socks can eat wherever they want.”
-- Reader’s Digest

1 December 1975, The Express (Lock Haven, PA), pg. 10, col. 2:
Watch that syntax!...Catholic Digest’s Kenn Maloney tells about the sign at Georgetown University which reads: “Shoes Are Required to Eat in the Cafeteria.”...Underneath, someone scribbled: “Socks May Eat Where They Want To.”

6 June 1976, The Sun (Baltimore, MD), pg. SM30: 
Sign in a university cafeteria: “Shoes are required to eat in the cafeteria.” Someone wrote below: “Slacks can eat wherever they want.”

Google News Archive
22 November 1976, Prescott (AZ) Courier, “Quips & Quotes,” Magazine, pg. ?, col. 2:
A sign on a university bulletin board reads: “Shoes Are Required to Eat in the Cafeteria.”
Underneath, somebody had scribbled, “Socks may eat wherever they want to.”
-- Dorothea Kent

Google Books
Fractured English
By Richard Lederer
New York, NY: Pocket Books
1996
Pg. 81:
At the entrance of a college cafeteria: SHOES ARE REQUIRED TO EAT IN THE CAFETERIA. Penciled underneath: SOCKS CAN EAT ANYWHERE THEY WANT.

TheTrucker.com
Joke Of The Day
Say what?
The Trucker Staff
7/12/2009
(...)
Sign in a cafeteria: “Shoes are required to eat in the cafeteria. In pencil beneath the sign: Socks can eat anyplace they want.”

San Diego (CA) Union-Tribune
Give us this season just 2 more wins
Written by Tom Blair‎
4:04 a.m., Dec. 19, 2010
(...)
Evan Bostich, a veteran of the grammar police, wonders at the sign he saw posted outside a North County diner: “Shirts and shoes are required to eat inside.” “So why,” he asks, “can’t shoes and shirts eat outside?”