When Firebase detects that you have insecure rules for a database, you are added to a queue along with other users who are using the same insecure rules, so you can be notified not to leave those rules unsecured. So you'll always get such a warning message.
If you are added to the queue and in the meantime you have changed the rules, then you'll still get the message, but you can ignore it, because your rules are secured.
If that's not the case, then it means that the message was sent due to other insecure rules. So please check if it's about this database or another one. If you want to disable such messages, as @FrankvanPuffelen already mentioned in his comment, you can disable the message directly in the Firebase Console: