Interesting, it is true that a simple text box is the easiest, fastest input method making it the most likely to use when in a rush/taking quick notes. I would have gone for something like workflowy (https://workflowy.com/) or dynalist (https://dynalist.io/) but having a single tool for both IM and note taking beats it. You could always have a bot that puts your slack content in those anyway. I shall take inspiration from this.