I'm curious as to what sort of reasoning went behind Kird Ape being too strong for Extended play? Granted this was many years ago but I was surprised to see that it was banned in a format that allowed powerful cards like Necropotence.
The simple answer is that at the time of the banning, WotC saw this card as a problem for the metagame in Extended. It could put an opponent pretty far behind pretty early, and many decks had to tailor early plays just to deal with Kird Ape. As Mark Rosewater notes in an article from 2003 (emphasis mine):
We ban and restrict cards because we believe there is something worse than not allowing players to use a particular card, and that is having a play environment become so degenerate that the game is no longer fun. Sometimes individual cards have to be sacrificed for the needs of the larger game.
When an individual card takes over a format because it seems unbalanced, WotC bans it in order to try and change the metagame. This is constant battle for them in the Modern format, where they're trying to figure out a way to ensure it's a four-turn format.
If it's any consolation, in the same article by Rosewater he admits the Kird Ape ban may have been a mistake:
Why did you ban Kird Ape in the original Extended format when Hypnotic Specter was not?
I’ll let you in on a little secret. We’re not infallible. We occasionally, gasp, make mistakes. We correct them when we catch them, but hey nobody’s perfect. Was the above a mistake? To quote my grandfather: “If it looks like a duck, quacks like a duck, and has a giant sign on it that says: ‘I’m a freakin’ duck!’, odds are, it might be a duck.”
I'm also skeptical that you are going to find a direct quote online referencing the Kird Ape ban. The earliest articles on the WotC website for Magic are from December 2001. I legitimately can't figure out when Kird Ape was actually banned and then unbanned, but this timeline shows Hypnotic Specter being banned in October of 1997. Since the above Rosewater quote references the banning of Kird Ape instead of Hypnotic Specter, I can only assume Kird Ape had both its banning and unbanning before then, more than four years before WotC decided to start using the internet to disseminate information about their game.
This is just speculation and my thoughts about it but I think that at that time a 2/3 for 1 mana was really very strong. I remember myself when they printed Isamaru, Hound of Konda and everyone was like, wow a 2/2 for 1 mana, (so strong that it had to be legendary). And even cards like Watchwolf people really liked, because also at that time you simply didn't get a 3/3 for 2 mana
Mostly, the given reason for the ban was that it was too good of an agro card. It outclassed every other 1 drop, and likely was better than many 2 drops as well. Of course, Wizard never really knows which cards are completely broken (ala Necropotence), so after time it was unbanned.