Collapsible sections are kind of already implemented. I have been using the encryption feature for this. After encrypting note content, it collapses to a one line block, then you can reveal/hide it as you wish, but you do have to give it a password. The password can be as simple as one character and only required once until you close the app; you don't have to enter it every time you reveal/hide the content. A limitation is that you can only have text in the section. It can be formatted text, but no files, tables, bullet lists, or other types of content.