Rockstar's policy is that you're free to mod the single-player game, but any modding whatsoever of the online game is forbidden, no matter how miniscule. They detect it and it's a potential ban.

That said, they don't publish anything about modding and don't have an official API available. Everything has been done third-party.

The modding has caused real problems in the online play - player in godmode, handing out in-game money, etc. No matter the efforts of R*, they seem to be unable to eradicate it for good. The group of players I previously played the game with, including myself, have all become extremely disenchanted with the game by the online modders.

It ultimately boils down to a cat and mouse game. What you end up with are extremely intrusive anticheat tools that do all sorts of nasty Trojan-like things; for example, injecting DLLs into a player's host machine to determine if they have modified the memory of the game.

The anticheat process itself will hide and run in kernelmode. I don't know about you, but I don't feel terribly comfortable having a closed source binary running in ring0.

I'm more comfortable with an anti-cheat blob running in ring0 than I am game-mod code running in any ring.

But, I run on a machine that was purpose-built for running games and if it all goes to hell, I just wipe and reload. All the games are from Steam, so they're easy to get back on there (and I only play a handful of those, so ...).

Nor do I, though it should be noted that most of the games with intrusive anti-cheat are Windows-only so the entire kernel is already closed-source blobs…