I think the last Zelda game that Impa was in was one of the Oracle games or Ocarina of Time. Other than that, she has not physically appeared in a lot of games like you said. Also, I don't want the Zelda series to be just be represented by Twilight Princess, so Sheik would be a very welcome addition.
Zelda's profile was put up in late June, yet there has not been an update for her since then, why? If Sheik did get cut, Sakurai would not have procrastinated as long as he has to deconfirm her. There was a lot of debate as to whether Lucas would be taking Ness's moveset, and Sakurai eventually did answer that question about that a little over a month after Lucas's profile was put up on the Dojo. It has been over five months since he put up Zelda's profile, yet he has yet to show anything on her. As a matter of fact, she is the only character on the Dojo that has not received an update. Also, every special-move of Zelda has been confirmed through some sort of video footage except for her Down-B. I think that Sheik will either be still activated via Down-B, or through a Final Smash. CoroCoro cannot come soon enough...