You probably know that the newest update to GTA V, 1.28, which contained the second part of the popular Ill-Gotten Gains DLC was causing a plethora of problems on the PC due to Rockstar's new anti-cheating mechanism. It included a large amount of dead code, making it harder for modders to find those portions of code that they need to get their mods working.
However, this mechanism affects scripts globally, meaning the code of the game proper, and increases processing time, essentially killing performance even with an unmodded version of GTA V running on a particularly beefy PC. Naturally, players were pretty pissed when GTA V started chugging on their 980s and Titans, not to mention that the development of rather popular mods was delayed due to the modders having to sift through a ton of dead code.
Rockstar's position on modding is unchanged. As long as you keep your mods in singleplayer, you're good to go, but they won't sacrifice the safety of GTA Online in order to make modding easier in story mode.
Our primary focus is on protecting GTA Online against modifications that could give players an unfair advantage, disrupt gameplay, or cause griefing. However, as a reminder, mods are still unauthorized and as such, Title Updates may cause Story Mode mods to behave in unexpected ways because they are not supported or tested, and players run them at their own risk.
The new patch, while fixing the performance issues in unmodded instances of the game, has once again broken most mods, so if you have any modifications running, expect to be downloading an updated, working version soon (provided they're still supported — if not, you are out of luck). Many PC players have called out Rockstar on not shipping GTA V and GTA Online as two separate executables, as that simple change would solve the issue of GTA Online security measures conflicting with GTA V mods.
