The project, which allows users to play GTA V with custom multiplayer modes on separate servers from Rockstar's own, has been active for a few months now. Under the impression that they were operating within the boundaries of Rockstar's ToS, the creators announced the mod to be safe and something that would avoid any repercussions from Rockstar. However, today it was confirmed that the bans were issued as a result of the FiveM mod, and that it certainly does violate Rockstar's policies:
"The FiveM project is an unauthorized alternate multiplayer service that contains code designed to facilitate piracy. Our policy on such violations of our terms of service are clear, and the individuals involved in its creation have had their Social Club accounts suspended."
So no matter how careful these modders were, trying to work within the present limitations, it seems they made a big mistake by creating a server framework that might allow connection for players using pirated games. Rockstar has made that perfectly clear now.
Source: PC Gamer
If you are using this mod, you might want to be careful.