>>44753328viridian forest was removed because of tech limitations and because there was no need to have a forest dungeon full of lvl 3 insect types this late into the game.
most of the areas that would have been dungeons in RBY were removed or shortered in GSC because they no longer appeared at the middle/end of the game and were thus redundant. they were the perfect things to remove for the GBC hardware/memory limitations.