Adding multiplayer to a game like Block Story is a massive undertaking because it requires rewriting the entire foundation of the game: Terrain Synchronization
While a standard, downloadable "multiplayer mod" for the main game does not officially exist on platforms like the Steam Workshop, several adjacent projects and theories have emerged: block story multiplayer mod
Creating a multiplayer mod for the original game is exceptionally difficult because: Engine & Code: Adding multiplayer to a game like Block Story