It can be enabled by swapping it with an existing melee weapon in the game files. However, it is normally disabled and cannot be found anywhere. On the other hand, the skateboard does exist as a fully functional weapon except in the PS2 version, where only the HUD icon remains. In the gta3.img file, the animation file for the skateboard exists. The skateboard is developed enough that it is specified to which surface is skateable.
There is little leftover data related to riding the skateboard. The Skateboard was a weapon and vehicle that was dropped during the development of Grand Theft Auto: San Andreas. It was meant to be introduced to the player in the mission Madd Dogg's Rhymes.
