You can find it here, among all the other options, -shadowQuality (0-3). 0 is for no shadows. It's official by R*, and the game picks this setting up in the menu. So how is this modding?

https://support.rockstargames.com/articles/202518358/Available-Command-Lines-for-GTAV-on-PC

Limiting frames with 3rd party soft is allowed, I've also seen ahk's on Discord. So why no shadows, something that R* made, is not allowed? It's a good way to boost 5-10 fps. In what way does it impact the gameplay itself? If I would make some weird custom resoultion, would it also not be allowed?

Honest question. Maybe I'm just stupid and don't see the obvious.

Graphical settings have to be selected in-game, command lines are not part of the in-game menu obviously. Changing settings without going through the in-game menu would give a performance advantage over a comparable PC that plays at in-game minimum settings, thus not being a fair competition. While it does seem like an unfair limitation to someone with a struggling computer, there are unfair advantages with unofficial settings, such as very bright lighting due to lack of shadows where it would normally be dark, and abnormally bright nighttime shootouts. "No shadows" looks terrible as well, and wouldn't make watching the runs enjoyable to the audiences of speedrunners, so it makes sense to ban a slight advantage that sacrifices much of the game's visual quality. The use of frame limiting software was never banned because GTAV handles high FPS poorly, with 150+ causing major stutters, plus there was once an FPS cap implemented, so banning the software that was once required to submit a run would be foolish. Autohotkey scripts are only allowed if it's a 1:1 rebind, not too much of an advantage over in-game rebinds, it's also hard to ban keyboard software rebinds, for obvious reasons.

"Graphical settings have to be selected in-game, command lines are not part of the in-game menu obviously. Changing settings without going through the in-game menu would give a performance advantage over a comparable PC that plays at in-game minimum settings, thus not being a fair competition. While it does seem like an unfair limitation to someone with a struggling computer, there are unfair advantages with unofficial settings, such as very bright lighting due to lack of shadows where it would normally be dark, and abnormally bright nighttime shootouts."

What advantage? Anyone can do that. Doesn't matter what PC you have. Guys that run the game on high end PC's with 140 fps, this is advantage. Of course I'm not complaining about expensive PC setups, I would be stupid to do so. It's just an example.

I played with no shadows for a while, and while it does look terrible, I don't think it gives any advantage at night. I'd say sometimes it's even a disadvantage. You have a feeling everything merges into one picture. If I set black depth (or however it's called in english) in my monitor settings, I can make the game very bright, without destroying the gamma completely. This is more advantage than noshadows.

""No shadows" looks terrible as well, and wouldn't make watching the runs enjoyable to the audiences of speedrunners, so it makes sense to ban a slight advantage that sacrifices much of the game's visual quality."

This is the only thing that makes sense to me.