The combo that got it banned was sangan/witch of the black forest, and "Chaos Emperor Dragon - Envoy of the End".
You nuked everyone's field and hand with Chaos Emperor Dragon, but since Sangan or Witch of the black forest was on your field at the time, you could use it's effect to get yata-garasu into your hand. where you would then summon and attack with it, depriving your hand and boardless opponent of their normal draw. While pecking them to death, you get to keep drawing until you get something that can OTK them, and that they have no answer for.
At the time it was banned, there were very few cards that you could activate from the graveyard, if any at all, so there was very little way to break the lock, and many of the countermeasures to setting it up didn't exist either.
Most of the easy tricks to get the bird out and wipe the board have been stopped now with eratta. Specificaly, Chaos Enperer dragon, sangan, and witch of the black forest have all been erattaed. So it seems that it wold no longer be worthy of a ban.
But it's a card that prevents your opponent from getting answers. and that's what make it broken. IF it gets through, it's because your opponent doesn't have an answer. If your opponent doesn't have an answer, he can't draw one. If the opponent also has no threats on the board or in hand, that's game. Lockdowns are not fun to face. They are worse than an OTK, because with a OTK at least the game is over. Other locks exist that are nearly impossible to escape as well, but they only get banned if they are too easy to set up. The cards used for them also nearly always have use outside the lock.
Yata-garasu's sole purpose is to prevent people from answering your threats. It's worthless when it's not doing this. It's only good as part of a lock. That's why, even though it's not nearly as easy to get the full lock anymore, it's still banned, and will forever be so.