Current plan for this is to store the word history as a file or set of files, ideally in a location of the user's choosing. That way, the user can choose to store their words in iCloud Drive, Dropbox, or anything else supported by the Files app. (Not sure if this would work, or is possible without being in the App Store... needs research.)

If it works, this solution has a very high bird:stone ratio — it can be more robust than NSUserDefaults, enables syncing (#42) across devices, and gives users access to their data for exporting (#29) to wherever they want. The data format can accommodate metadata about each entry, enabling favourites, last-searched date (#14), etc.