Why did former National Football League player Doak Walker have one shoe with a zipper?

Ewell Doak Walker II (1927-1998) was both the 1948 Heisman Trophy winner and a professional football player in the NFL. He has been inducted into two football halls of fame: the College one in 1959 and the Pro version in 1990. The Doak Walker Award for the top running back in college football has been awarded annually since 1990 and bears his name.
https://wiki2.org/en/Doak_Walker+Newton

The answer to the question is for ease of quickly changing footwear during a match.

There were some players who could punt as well as placekick. But during the haste of a game, some of them would forget to change shoes for a punting situation. This then led to many a shanked punt. To avoid this situation, Doak Walker, who placekicked and punted for the Detroit Lions (1950-55), had a zipper sewn on the outside of this square-toe shoe. It enabled him to quickly change shoes especially for punting.

"Placekicking in the NFL: A History and Analysis" (2014, p. 23) By Rick Gonsalves, available on Google Books