Starting in Android 8.0 (API level 26), all notifications must be assigned to a channel. For each channel, you can set the visual and auditory behavior that is applied to all notifications in that channel. Then, users can change these settings and decide which notification channels from your app should be intrusive or visible at all.
After you create a notification channel, you cannot change the notification behaviors—the user has complete control at that point. Though you can still change a channel's name and description.