Kind of an impossible to answer question, really. Arguably in the original Legend of Zelda and in the Oracle games, Impa telling you what's going on isn't 100% necessary, and Link could probably figure out what he's doing on his own. In Zelda 2's backstory, the game itself probably would have never happened without Impa telling him to get the Triforce of Courage... but I'm not sure what that would really change. Most of the game is spent simply fighting Ganon's minions in an attempt to revive him... I'm not sure how Link finding the Triforce or not affects that.
It gets weird in Ocarina of Time and Skyward Sword. In Ocarina of Time, Impa isn't really all that important on her own. Yes she got Zelda out of Hyrule Castle when Ganon took over, but... probably someone else could have done that. Yes she was the Sage of Shadow, but... that's important because of the position, not Impa herself. So the question is, what would "without Impa" mean in this case? Did she die? Or did she just not exist? If the latter, there just would have been a different Sage of Shadow.
In Skyward Sword it's a similar story. Impa does some really important things in her saving Zelda, but arguably another servant or even another actual Sheikah could have done it instead.
She's basically both pivotal yet replaceable in every game she's in.
