Android 8 (API level 26) expects any notification to use a Channel. So we need to implement them but only when the API level is 26+. Channels should be ignored otherwise because they won´t be available, meaning that your app will throw an exception if you try to use them.

We will create a default channel to be used when any push notification arrives. You can create multiple channels but I bet most applications will be just fine with one.

public static class NotificationHelper 
{ 
    public const string ChannelId = "com.unique.string.onyourapp.default";

    // make sure you call this method only once 
    public static void CreateDefaultChannel(Context context) 
    { 
        if (Build.VERSION.SdkInt < BuildVersionCodes.O) 
            return;
        var channel = new NotificationChannel(ChannelId, 
            "AppName default", NotificationImportance.High);

        channel.EnableVibration(true); 
        channel.LockscreenVisibility = NotificationVisibility.Public;

        var notificationManager = (NotificationManager) context
            .GetSystemService(Context.NotificationService);

        notificationManager.CreateNotificationChannel(channel); 
    }

    // method extension to ease the creation of the nofication 
    public static NotificationCompat.Builder SetChannelIdIfNeeded( 
        this NotificationCompat.Builder builder, string channelId) 
    { 
        if(Build.VERSION.SdkInt >= BuildVersionCodes.O) 
            builder.SetChannelId(channelId);

        return builder; 
    } 
}

It's a good idea to create the default channel at the app start, either on your main activity or your custom Application OnCreate() override:

public override void OnCreate() 
{ 
    base.OnCreate(); 
    NotificationHelper.CreateDefaultChannel(this); 
}

Last, but not least, you need to make and slight modification to the code that creates the notification.

Before:

var notificationBuilder = new NotificationCompat.Builder(this)
    .SetSmallIcon(Resource.Drawable.ic_notification)
    .SetContentTitle(title)
    .SetContentText(message)
    .SetAutoCancel(true)
    .SetContentIntent(pendingIntent);

After:

var notificationBuilder = new NotificationCompat.Builder(this)
    .SetChannelIdIfNeeded(NotificationHelper.ChannelId) // method extension
    .SetSmallIcon (Resource.Drawable.ic_notification)
    .SetContentTitle(title)
    .SetContentText(message)
    .SetAutoCancel(true)
    .SetContentIntent(pendingIntent);

From now on your push notifications will start working again on Android 8.