Notification in status bar
Obviously, the operating system has a limited number of resources for itself and for the apps it is running. So, when the system needs to free up a bit to make everyone happy, it is trying hard to decide what is safe to unload right now. And it's where our notification comes for. It not only indicates that the recording is in progress, it tells the system that our background process is very important for the user and he'll not be happy if it will be terminated. And fortunately, it works.
Beginning from Android 9 to be able to use a microphone in the background it's actually required to show a notification. Otherwise, the app will not have any data from the mic. So for these reasons, we can't have an option to remove notification in the app anymore.