While I agree that contact with pre-warp civilizations was nothing unusual in TOS I think that TNG established it as something that was prohibited as early as "Who Watches the Watchers" in Season 3.
Revealing your existence to species that didn't already know about alien life was off-limits from TOS onward (see "Bread and Circuses" and "A Private Little War"), though inconsistently applied. The annoying mistake "First Contact" made was equating that with "pre-warp." It's pat and simplistic to assume that inventing warp drive was the only possible way a species could learn about other worlds. What about radio astronomy and SETI? What about space telescopes powerful enough to image exoplanets and identify biosignatures and technosignatures? Realistically, most technological civilizations should be able to confirm the existence of aliens long before they develop warp drive. And that's not even counting the possibility that non-Federation starfarers without a Prime Directive could have already contacted a pre-spaceflight world long before Starfleet discovered it. It's simplistic to assume Starfleet is the only game in town.