Rockstar's current official stance on mods is their statement released in 2017 following the OpenIV cease-and-desist reversal. Let's break down what it actually means, and what we can expect for GTA 6.
What the 2017 statement said
Single-player modding is usually tolerated as long as the mods don't contain copyrighted material, allow for cheating in multiplayer, contain advertisements, or make money directly off of modding. Modding online is not tolerated.
The statement created parameters that have held since 2017. It isn't a guarantee for GTA 6, but it's the most concrete position we've heard publicly.
How GTA 6 might be different
A bigger online component. There might be less separation between SP and online code paths. The Cfx.re acquisition alters the multiplayer-modding landscape.
Any of these could influence policy. None of them have been publicly addressed in relation to GTA 6 so far.