Xamarin.Forms Entry just with bottom border. Added IsValid and ErrorColor Properties to better handle Validation Scenarios

LineEntry.cs

using System;

using Xamarin.Forms;

namespace YOURNAMESPACE
{
	public class LineEntry : Entry
	{
        public static BindableProperty BorderColorProperty = 
			BindableProperty.Create(
				nameof(BorderColor), 
				typeof(Color), 
				typeof(LineEntry), 
				Color.Default, 
				BindingMode.TwoWay);
        public Color BorderColor
		{
			get { return (Color)GetValue(BorderColorProperty); }
			set { SetValue(BorderColorProperty, value); }
		}

        new public static readonly BindableProperty FontSizeProperty = 
			BindableProperty.Create(
				nameof(FontSize),
                typeof(double),
                typeof(LineEntry),
                Font.Default.FontSize,
                BindingMode.TwoWay);
        new public double FontSize
		{
			get { return (double)GetValue(FontSizeProperty); }
			set { SetValue(FontSizeProperty, value); }
		}

        new public static readonly BindableProperty PlaceholderColorProperty = 
			BindableProperty.Create(
                nameof(PlaceholderColor),
                typeof(Color),
                typeof(LineEntry),
                Color.Default,
                BindingMode.TwoWay);      
        new public Color PlaceholderColor
		{
			get { return (Color)GetValue(PlaceholderColorProperty); }
			set { SetValue(PlaceholderColorProperty, value); }
		}

		public static BindableProperty ErrorColorProperty =
            BindableProperty.Create(
                nameof(ErrorColor),
                typeof(Color),
                typeof(LineEntry),
				Color.Default,
                BindingMode.TwoWay);
        public Color ErrorColor
        {
            get { return (Color)GetValue(ErrorColorProperty); }
            set { SetValue(ErrorColorProperty, value); }
        }

		public static BindableProperty IsValidProperty =
            BindableProperty.Create(
				nameof(IsValid),
				typeof(bool),
                typeof(LineEntry),
                false,
                BindingMode.TwoWay);
		public bool IsValid
        {
			get { return (bool)GetValue(IsValidProperty); }
			set { SetValue(IsValidProperty, value); }
        }
	}
}

LineEntryRenderer_Android.cs

using System.ComponentModel;
using Android.Graphics;

using YOURNAMESPACE.Android;

using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(LineEntry), typeof(LineEntryRenderer))]

namespace YOURNAMESPACE.Android;
{
    public class LineEntryRenderer : EntryRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
        {
            base.OnElementChanged(e);
            if (Control == null || Element == null || e.OldElement != null) return;

			if(e.NewElement != null) {
				var element = (LineEntry)Element;
				DrawAll(element);
            }
        }

		protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);
            var view = (LineEntry)Element;

			if (e.PropertyName.Equals("IsValid") || e.PropertyName.Equals("ErrorColor"))
				DrawAll(view);
			if (e.PropertyName.Equals("BorderColor"))
                DrawBorder(view);
            if (e.PropertyName.Equals("FontSize"))
                SetFontSize(view);
            if (e.PropertyName.Equals("PlaceholderColor"))
                SetPlaceholderTextColor(view);
        }

		protected void DrawBorder(LineEntry element) {
			Invalidate();
			var color = element.BorderColor.ToAndroid();

			// If the element is invalid, and the ErrorColor has been set, change the color
			if (!element.IsValid && element.ErrorColor != Xamarin.Forms.Color.Default)
				color = element.ErrorColor.ToAndroid();
        	Control.Background.SetColorFilter(color, PorterDuff.Mode.SrcAtop);
		}

		protected void SetPlaceholderTextColor(LineEntry element) {
			Control.SetHintTextColor(element.PlaceholderColor.ToAndroid());
		}

		protected void SetFontSize(LineEntry element) {
			Control.SetTextSize(Android.Util.ComplexUnitType.Sp, (float)element.FontSize);
		}

		protected void DrawAll(LineEntry element) {
			DrawBorder(element);
            SetPlaceholderTextColor(element);
            SetFontSize(element);
		}
    }
}

LineEntryRenderer_iOS.cs

using System.ComponentModel;

using CoreAnimation;
using CoreGraphics;
using Foundation;
using UIKit;

using YOURNAMESPACE.iOS;

using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer (typeof(YOUTNAMESPACE.LineEntry), typeof(LineEntryRenderer))]
namespace YOURNAMESPACE.iOS
{
	public class LineEntryRenderer: EntryRenderer
	{
		protected override void OnElementChanged (ElementChangedEventArgs<Entry> e)
		{
			base.OnElementChanged (e);

			if (Control != null) {
				Control.BorderStyle = UITextBorderStyle.None;

				var view = (Element as LineEntry);
				if (view != null) {
					DrawAll (view);
				}
			}
		}

		protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
		{
			base.OnElementChanged(e);

			if (Control != null)
			{
				Control.BorderStyle = UITextBorderStyle.None;

				var view = (Element as LineEntry);
				if (view != null)
				{
					DrawBorder(view);
					SetFontSize(view);
					SetPlaceholderTextColor(view);
				}
			}
		}

		protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
		{
			base.OnElementPropertyChanged(sender, e);

			var view = (LineEntry)Element;

			if (e.PropertyName.Equals("IsValid") || e.PropertyName.Equals("ErrorColor"))
				DrawAll(view);
			if (e.PropertyName.Equals("BorderColor"))
				DrawBorder(view);
			if (e.PropertyName.Equals("FontSize"))
				SetFontSize(view);
			if (e.PropertyName.Equals("PlaceholderColor"))
				SetPlaceholderTextColor(view);
		}

		void DrawBorder(LineEntry element)
		{
			SetNeedsDisplay();
			var color = element.BorderColor.ToCGColor();

			// If the element is invalid, and the ErrorColor has been set, change the color
			if (!element.IsValid && element.ErrorColor != Color.Default)
				color = element.ErrorColor.ToCGColor();

			_borderLayer = new CALayer
			{
				MasksToBounds = true,
				BorderColor = color,
				BorderWidth = 1.0f
			};

			Control.Layer.AddSublayer(_borderLayer);
			Control.BorderStyle = UITextBorderStyle.None;
		}

		void SetFontSize(LineEntry element)
		{
			if (element.FontSize != Font.Default.FontSize)
				Control.Font = UIFont.SystemFontOfSize((System.nfloat)element.FontSize);
			else if (element.FontSize == Font.Default.FontSize)
				Control.Font = UIFont.SystemFontOfSize(17f);
		}

		void SetPlaceholderTextColor(LineEntry element)
		{
			if (string.IsNullOrEmpty(element.Placeholder) == false && element.PlaceholderColor != Color.Default)
			{
				var placeholderString = 
					new NSAttributedString(
						element.Placeholder, 
						new UIStringAttributes { 
    						ForegroundColor = element.PlaceholderColor.ToUIColor()
    					});
				Control.AttributedPlaceholder = placeholderString;
			}
		}

		protected void DrawAll(LineEntry element)
		{
			DrawBorder(element);
			SetPlaceholderTextColor(element);
			SetFontSize(element);
		}
	}
}

Hello

The ios worked with some tweaks but the android did not, see attached