This rule was possibly published in 2007, in an expanded edition of the Rules of the Internet.
A definition was admitted to Urban Dictionary in the same year.
Though Rule 63 is nearly not as popular, as Rule 34, it is still widely known and recognized all around the web, from 4chan to Reddit, from Tumblr to Instagram and Facebook.
On DeviantArt, the phenomenon has been present since 2009, with heaps of artworks being tagged with the rule.
In the same year, a site was also dedicated to Rule 63, where people could genderswap their favorite characters and share the artworks.
Comic book conventions host thousands of examples every year for Rule 63, with fans hopping into the bodies of their preferred heroes and heroines, regardless of their own gender, creating characters, such as the Female Joker.
Futurama also featured the idea in their Season 7 premiere, where characters were presented as the opposite gender versions of themselves.