I've been slowly rewatching TOS and I have to say I don't understand how Assignment: Earth can be considered canon. It's a backdoor pilot for an unrelated concept. It treats time travel casually (Enterprise was given as assignment to study history!?). It is never referenced again (outside of the same method of time travel being used in STIV). Frankly it's the most logically incongruent episode in the entire franchise in terms of canon.