But most of Geany use-cases are about coding, where line breaking is not wanted, so thats probably why its never been done.
I don't buy this argument at all because while coding you usually do not cross the line limit but reformat the code before getting there - and if you don't the autobreaking might be a well-deserved reminder that you actually should do that. The only reason why you would cross it is writing long comments and that's exactly where auto line breaking (with auto-continuing comments) would beneficial to many users.