First slash fan fiction, since slash ships started with the ST fandom, with Kirk/Spock (or K/S, also known as "The Premise") specifically. Published in the zine "Grup" #3.
note: slash was used before people started mixing characters's names or creating new names for ships. I don't use AO3, but I'm aware slash is still used there.
"Mary Sue" archetype's name origin.
The first Star Trek's fan stories, printed in fan zines specifically, were the ones from Spockanalia's issue #1, from 1967.