In a real sense, every Trek story is about an exploration or discovery of some kind. The opening monologue in TOS and TNG about "exploring strange new worlds and seeking out new life and civilizations" really happens in every episode--as far as modern-day viewers are concerned
, IMO. In any event, it was never taken quite so literally in either TOS or TNG, because if anything exploration was just used as an excuse for the Enterprise
to be out there and actually do a wide variety of things besides literal space exploration.