Custom skins, Huds, character mute mods, etc. Mods that do not change the mechanics/difficulty of the game are harmless and most people won't even know that you're using them. These types of mods are generally best kept to your own lobby, set to Private or Friends Only but you won't get in trouble. are considered cheating by some players and not considered cheating by others, but they are not outright 'illegal.' As long as you're not ruining others' gaming experience by forcing your gameplay preferences on people who don't want it, you should be fine. like CarryStacker (lets you carry more than one loot bag at a time), SilentAssassin (lets you change/circumvent the Pager system), BetterBots (adjusts the stats/behaviour of bots to make them more effective than not using mods), etc. Mods that change the mechanics and potentially make the game easier, but don't fall under the category of 'obviously cheating'. Unlocking achievements without earning them and piracy (unlocking content that normally requires payment to use) are also very much against the rules.

Mods that give you godmode, give you infinite ammo, spawn lootbags, or instantly complete heists are considered cheating by the developers and can get you in trouble. ^ As the developers have said, common sense is the best guide. Originally posted by Optimist Prime (the Gray):

Payday 2 beyond cheats trainer