There was some cut content left over but much more that didn't make it into any public build.
Originally Sonic 2 had time travel mechanics that they spun off to make Sonic CD, some of the stages still in Sonic 2's retail release were actually conceived as alternate past and future versions of the same zones before being reworked.
The progression they plan is always supposed to be that you start out in rural forests or plains and at the end transition into the mechanized settings where you confront Eggman, but they always ended up scrambling the order of the levels.
Chemical Plant in Sonic 2, was originally at the end of the game, and might have been planned as a full zone for the original death egg, iirc. Got put near the game's start because it ended up being too easy and Aquatic Ruins was too difficult following Emerald.
There's a Sonic 2 remaster for smartphones by the lead devs that went onto develop Mania, that restored the cut Hidden Palace Zone as a hidden easter egg, while also keeping Sega's w.i.p. version of the level hidden in the sound test.