No. Instagram has never "sent" this kind of notification to it's users, and the current Instagram App does not send this notification. To understand situations why you or a recipient may have seen it in the past, imagine your message recipient with a locked phone screen that has your unseen message notifications on it:
1. On older versions of the Instagram App, your unseen message notifications sitting on a recipient's lock screen would get replaced by a 'User unsent a message' notification. So instead of seeing your message in the notification, the user would see 'User unsent a message' instead.
2. On the latest version of the Instagram App, your unseen messages notifications sitting on a recipient's lock screen will disappear one-by-one in the order they are unsent.
3. On all versions of the Instagram App that we tested, already seen notifications will NOT be replaced with, or create new, `User unsent a message` notifications at all.
Since most people are in and out of their phones and clearing notifications from their lock screens frequently, it's very unlikely that anybody will ever see this message, and they won't see it at all if they have already seen your messages or if they have the latest version of the Instagram App. To date, none of our customers have reported people receiving this message during the unsending process with UnSendAll.