public boolean areNotificationsEnabled() {
        if (Build.VERSION.SDK_INT >= 24) {
            return mNotificationManager.areNotificationsEnabled();
        } else {
            AppOpsManager appOps =
                    (AppOpsManager) mContext.getSystemService(Context.APP_OPS_SERVICE);
            ApplicationInfo appInfo = mContext.getApplicationInfo();
            String pkg = mContext.getApplicationContext().getPackageName();
            int uid = appInfo.uid;
            try {
                Class<?> appOpsClass = Class.forName(AppOpsManager.class.getName());
                Method checkOpNoThrowMethod = appOpsClass.getMethod(CHECK_OP_NO_THROW, Integer.TYPE,
                        Integer.TYPE, String.class);
                Field opPostNotificationValue = appOpsClass.getDeclaredField(OP_POST_NOTIFICATION);
                int value = (int) opPostNotificationValue.get(Integer.class);
                return ((int) checkOpNoThrowMethod.invoke(appOps, value, uid, pkg)
                        == AppOpsManager.MODE_ALLOWED);
            } catch (ClassNotFoundException | NoSuchMethodException | NoSuchFieldException
                    | InvocationTargetException | IllegalAccessException | RuntimeException e) {
                return true;
            }
        }
    }
Returns whether notifications from the calling package are not blocked.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/compat/NotificationManagerCompat.java