TOS would occasionally make direct reference to past episodes, though. Kirk, for example, advises Spock to use the same telepathic trick that he used in "A Taste of Armageddon" on the Kelvans in "By Any Other Name." In the same episode, he also indicates that the Enterprise had already visited the Galactic Barrier, a reference to "Where No Man Has Gone Before."
TNG did this more, though. (And DS9 did it more than TNG).