Really, this post shouldn't need to be published, but there are such an overwhelming number of questions in various GTA Online related fan communities about certain types of mods being allowed or not that it has become necessary. There is a very clear, easy to understand and all-inclusive guideline to modding in Online.
While most players get that mods that give them an obvious unfair advantage aren't allowed, there are a typical few types of mods that keep coming up in questions along the lines of "is this an exception?" or "will this also get me banned?". Here's the thing: there are no exceptions, and all mods will get you banned.
Some of the most frequent offenders are FOV mods, graphics mods and real-life car mods. Now, while it may not be apparent, all of these affect gameplay in a way that imbalances the relationship between modded player and unmodded player.
The first one, the Field of View mod is the most obvious one. There's this thing where PC players (I too am guilty of this) are bummed out when the FOV in a game - meaning how wide your field of view extends around the character - is capped at 90 and cannot be adjusted.
Now, seeing as GTA 5 can be modded, FOV mods are there to solve this in single player. However when you go Online, things change. If you use an FOV mod, you'll have a direct advantage over people who don't, as you could potentially see them without them seeing you even if you were in the same relative position to one another. Seeing around corners others can't is another advantage posed by these mods.
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Graphics mods also come up. Why would higher-resolution texture, better lighting effects and so forth give you an edge? It's simple, really. Visual identification is key in shootouts, as is keeping track of your targets. Different lighting effects could alter the way shadows fall for you as opposed to other players, while higher draw distances mean you'll see an opponent far sooner than they will see you.
Better textures, higher draw distances and higher quality models will allow you to more accurately identify weapons and vehicles used by the enemy at a distance, allowing you to tailor your tactics accordingly. Better reflections could tick you off on a sneak attack which otherwise wouldn't be obvious. There really are a myriad of ways that graphical mods can tip the scales in your favor. Hence, they are also banned.
Finally, with car mods, changing the appearance of vehicles to resemble real world cars or even other non-GTA fictional vehicles is the least apparent way of gaining an advantage. First, you need to understand that GTA Online being a peer-to-peer connection, most things that happen in game do so client-side, so third party files in your game directory will only affect your screen. Other players won't see some car as a Batmobile if they don't have the mod installed.
Depending on how sophisticated a mod you're using, the modded cars might have their own unique damage states mapped, causing different deformations, resulting in different forces acting in different directions, which would whack the physics engine of the game right home. Using such mods that change in-game models won't even let you boot up GTA Online anymore, as they will throw you an incompatibility exception.
Basically, the rule of no exceptions would apply even if a mod wouldn't grant you an advantage. The newest anti-modding systems GTA Online uses will detect any third party tampering and ban you, regardless of the mod.
Spread the word - Absolutely no modding in GTA Online.