The only original Trek episodes that I'm pretty certain have not been followed up in some way are "Spock's Brain" and "Wink of An Eye".
Every other one I can think of has had characters that appeared in them appear in a book or comic somewhere or had the story followed up in some way.
Several more than once.
Every other one I can think of has had characters that appeared in them appear in a book or comic somewhere or had the story followed up in some way.
Several more than once.