Intercepts the "soft" back button events from the Android toolbar and forwards them to the Page.OnBackButtonPressed for handling.

InterceptBackButtonNavigationPageRenderer.Android.cs

using System.Reflection;
using System.Threading.Tasks;
using Android.Content;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android.AppCompat;
using AToolbar = AndroidX.AppCompat.Widget.Toolbar;

[assembly: ExportRenderer(typeof(NavigationPage), typeof(MyApp.Droid.Renderers.InterceptBackButtonNavigationPageRenderer))]
namespace MyApp.Droid.Renderers
{
    public class InterceptBackButtonNavigationPageRenderer : NavigationPageRenderer
    {
        private readonly FieldInfo _toolbarFieldInfo;
        AToolbar Toolbar => _toolbarFieldInfo.GetValue(this) as AToolbar;

        private readonly FieldInfo _flyoutPageFieldInfo;
        FlyoutPage FlyoutPage => _flyoutPageFieldInfo.GetValue(this) as FlyoutPage;

        public InterceptBackButtonNavigationPageRenderer(Context context) : base(context)
        {
            _toolbarFieldInfo = typeof(NavigationPageRenderer).GetField("_toolbar", BindingFlags.Instance | BindingFlags.NonPublic);
            _flyoutPageFieldInfo = typeof(NavigationPageRenderer).GetField("_flyoutPage", BindingFlags.Instance | BindingFlags.NonPublic);
        }

        protected override void OnAttachedToWindow()
        {
            base.OnAttachedToWindow();

            BindSoftNavigationInterception();
        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            base.OnLayout(changed, l, t, r, b);

            BindSoftNavigationInterception();
        }

        private void BindSoftNavigationInterception()
        {
            Toolbar.NavigationClick -= Toolbar_NavigationClick;
            Toolbar.NavigationClick += Toolbar_NavigationClick;
        }

        private async void Toolbar_NavigationClick(object sender, AToolbar.NavigationClickEventArgs e)
        {
            if (Element is null)
            {
                return;
            }

            if (Element is INavigationPageController controller
                && controller.StackDepth <= 1)
            {
                if (FlyoutPage != null)
                {
                    FlyoutPage.IsPresented = !FlyoutPage.IsPresented;
                }
            }
            else
            {
                Element.SendBackButtonPressed();
            }
        }

        protected override async Task<bool> OnPopToRootAsync(Page page, bool animated)
        {
            var result = await base.OnPopToRootAsync(page, animated);

            BindSoftNavigationInterception();

            return result;
        }

        protected override async Task<bool> OnPushAsync(Page view, bool animated)
        {
            var result = await base.OnPushAsync(view, animated);

            BindSoftNavigationInterception();

            return result;
        }

        protected override async Task<bool> OnPopViewAsync(Page page, bool animated)
        {
            var result = await base.OnPopViewAsync(page, animated);

            BindSoftNavigationInterception();

            return result;
        }
    }
}