The Discoverable by Others option, quietly enabled, allows other iPhone users with the Journal app to detect your iPhone via Bluetooth. Their Journal app uses that data to improve their Journal suggestions, as your Journal app will do for you when you get close to other iPhones in the wild.

When you head to the Settings app, choose “Privacy & Security” then tap “Journaling Suggestions,” you’ll see a “Discoverable by Others.” This is enabled by default, regardless of whether the aforementioned “Journaling Suggestions” feature is enabled.

"Discoverable by Others" lets iOS detect how many other devices are nearby, but does not share personal info. Apple encrypts Journal data for user privacy, and the discoverability feature can be turned off if desired in settings. Apple has added a setting to iOS called "Discoverable by Others".