How do we, as fans, deal with new elements introduced in Trek? Do we assume the new element is new in and out of universe? Or do we assume the new element is an old element never before seen?
The DS9 jumpsuits were not seen anywhere
before their introduction in "Emissary". That's because they were new to DS9, to visually distinguish it from TNG. In-universe, though, what do we say?
Do we say that to commemorate the acquisition of DS9, Starfleet issued new uniforms for officers assigned to DS9 to wear? That doesn't strike me as realistic, in that such a large organization would probably not care about a minor facility such as DS9.
So the alternative is to assume the new element was always available but unseen. The DS9 jumpsuits are some sort of less formal work attire available to Starfleet officers, and given the gritty environment of DS9, a uniform you wouldn't be afraid to get dirty in was needed. Sisko, as a former engineering officer, decreed the jumpsuit to be the duty uniform.
Could they have laid some foundation for the DS9 jumpsuits previously? Absolutely. There were plenty of times that the TNG crew needed to wear something more work-oriented than the office wear uniforms they donned most of the time. There were a couple of jumpsuits that were seen in TNG, such as the pic that Vasquez Rocks
linked to. The DS9 suits seem similar to that, with colors reversed, though (and division colors on the shoulders). We could consider that a precursor to the DS9 uniforms.