From Bulbapedia, the community-driven Pokémon encyclopedia.
During the planning and development stages of Pokémon FireRed and LeafGreen, several parts were going to be a part of the games. Some of them were dropped or changed, but some of those dropped concepts of the beta version can still be found in the game's programming.
The islands in the Sevii Islands region would likely be named Sevii Isles. In addition, several more locations in the Sevii Islands have been found in the programming code, including Sevii Isles 6-9 and 22-24. It is very notable that Sevii Isle 8 and 9 have fully coded map data, implying that they were scrapped very late in the game's production.
Key items from Ruby and Sapphire exist within Firered and Leafgreen programming. They have little or no function at all. For Example: the Mach Bike works but only moves as fast as the bike in Firered and Leafgreen. An example of an item that does absolutely nothing whatsoever is Devon Goods. When selected Oak says it is not time to use it. These items can be obtained via Gameshark SP mod shop key items code. This code can no longer be obtained in its original form on Gmeshark.com but other websites have the originals posted for both Firered and Leafgreen versions 1.0 and 1.1. These items are presumed as placeholders which hints towards a possiblibility that Firered and Leafgreen were built off of the Ruby and Sapphire game engine. There is also HM08 present in the coding as well.