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The Temporal Prime Directive was a fundamental Starfleet principle.
All Starfleet personnel were strictly forbidden from directly interfering with historical events and were required to maintain the timeline and prevent history from being altered. It also restricted people from telling too much about the future, so as not to cause paradoxes or alter the timeline.
Lieutenant Ducane told Kathryn Janeway, "Remember the Temporal Prime Directive: discuss your experiences with no one," implying that it was the wording consistent with the Temporal Prime Directive in the 29th Century (at least in that timeline).
The Temporal Prime Directive was directly related to the Prime Directive.