Apple SMS has been muted. Was it hacked?

SMS has muted the notification, not hacked it.

Apple's SMS mute function is to let users control the notification volume or mute the notification, instead of being blacked out by the other party. If the other party hacked himself, he could not communicate with the other party, including sending text messages and making phone calls. When you receive the message "notification has been muted", it only means that the other party's notification is set to mute, not blacked out.