The main story line in Borderlands 3 is pretty long, but there are still many people that want to keep playing the game after completing everything. So, does Borderlands 3 even have a New Game Plus? Yes, it does indeed. However, it's not called New Game+ in the Borderlands franchise, it's called True Vault Hunter Mode. It is effectively the same thing though.
When you start a True Vault Hunter Mode, all your levels, skill, gear, weapons, money etc. is transferred with your character, but you begin the rest of the game from the beginning. All the enemies also level up, and get a lot more difficult. This allows you to level up even higher yourself, face more difficult enemies, and of course get better loot!
The main story mode in True Vault Hunter mode is exactly the same as the first play though, so don't expect to do anything new in terms of story here. However, all the missions and quests are reset, so you get to complete them all over again.