iPhone, iPad, and Apple Watch devices have a special feature that tracks how loud the music is for their users. This alert is designed to protect them from sound that may permanently affect their hearing.
If you are wearing headphones, your iPhone can warn you when you “exceed 100% of the 7-day exposure limit” When you receive the notification, it will ask you to lower the volume. In some countries and regions, it is impossible to turn on these notifications. Apple says this is due to regulations and safety standards.