Which stage was planned to be a part of "Sonic the Hedgehog 2", but was scrapped during development?

Genocide City Zone was planned to be a part of Sonic the Hedgehog 2 but was ultimately scrapped during development due to time constraints and the developers' concerns that the name and theme of the level were too dark and potentially controversial for a game aimed at a younger audience. The creators felt that it would be more appropriate to replace it with the less intense and more vibrant level, "Cyber City Zone," to maintain a lighter tone in line with the rest of the game.