2 chan was really the "original" imageboard, it was more Japanese based, 4 chan is the one that Moot made that really took off on a global level.
Anyways, the board that should not be mentioned is certainly not illegal to browse so long as you don't post illegal content or possibly save illegal pictures.
Don't know why you wouldn't want to visit it every once in a while. If you have the stomach for it, its mostly shitposting with some higher level memes poppnig out once in a while.