A search-by-username feature sounds simple but raises a real design question: whether the app should let people be found that way, because it changes the balance between discovery and control. Most users probably want the option to be findable by people they already know, but not by everyone. What tends to work in these situations is a toggle rather than a global setting — default to private, let people opt into searchability — because the users who most need protection are the least likely to find a buried setting. There's a discussion on https://thorfortunes.ca about privacy features that work by default rather than by configuration — the point about defaults determining outcomes was well-made. Would you want that?