diff --git a/Documentation/guides/messages/xa0121.md b/Documentation/guides/messages/xa0121.md
new file mode 100644
index 00000000000..c736263a2f1
--- /dev/null
+++ b/Documentation/guides/messages/xa0121.md
@@ -0,0 +1,34 @@
+---
+title: Xamarin.Android warning XA0121
+description: XA0121 warning code
+ms.date: 09/19/2019
+---
+# Xamarin.Android warning XA0121
+
+## Issue
+
+The behavior implemented in the
+`<GetAdditionalResourcesFromAssemblies/>` MSBuild task is now
+deprecated.
+
+This MSBuild task is a precursor to [Xamarin.Build.Download][xbd] that
+enables downloading of Android packages from the internet.
+
+Libraries using any of the following custom assembly-level attributes
+will encounter this warning:
+
+* `IncludeAndroidResourcesFromAttribute`
+* `NativeLibraryReferenceAttribute`
+* `JavaLibraryReferenceAttribute`
+
+## Solution
+
+The [Xamarin Support Libraries][supportlibs], can be simply updated to
+a newer version on NuGet.
+
+Library authors will need to remove usage of these deprecated
+attributes. Their functionality will be removed in a future version of
+Xamarin.Android.
+
+[xbd]: https://www.nuget.org/packages/Xamarin.Build.Download
+[supportlibs]: https://github.com/xamarin/AndroidSupportComponents
diff --git a/src/Mono.Android/Android/IncludeAndroidResourcesFromAttribute.cs b/src/Mono.Android/Android/IncludeAndroidResourcesFromAttribute.cs
index e310b7a4123..e89125dfd86 100644
--- a/src/Mono.Android/Android/IncludeAndroidResourcesFromAttribute.cs
+++ b/src/Mono.Android/Android/IncludeAndroidResourcesFromAttribute.cs
@@ -3,6 +3,7 @@
 namespace Android {
 
 	[AttributeUsage (AttributeTargets.Assembly, AllowMultiple = true)]
+	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
 	public class IncludeAndroidResourcesFromAttribute : ReferenceFilesAttribute
 	{
 		public IncludeAndroidResourcesFromAttribute (string path)
diff --git a/src/Mono.Android/Android/NativeLibraryReferenceAttribute.cs b/src/Mono.Android/Android/NativeLibraryReferenceAttribute.cs
index f45f95e76eb..88941e5e649 100644
--- a/src/Mono.Android/Android/NativeLibraryReferenceAttribute.cs
+++ b/src/Mono.Android/Android/NativeLibraryReferenceAttribute.cs
@@ -3,6 +3,7 @@
 namespace Android {
 
 	[AttributeUsage (AttributeTargets.Assembly, AllowMultiple = true)]
+	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
 	sealed public class NativeLibraryReferenceAttribute : ReferenceFilesAttribute
 	{
 		public NativeLibraryReferenceAttribute (string filename)
diff --git a/src/Mono.Android/Android/ReferenceFilesAttribute.cs b/src/Mono.Android/Android/ReferenceFilesAttribute.cs
index 6d98891f5de..c73ebc394a2 100644
--- a/src/Mono.Android/Android/ReferenceFilesAttribute.cs
+++ b/src/Mono.Android/Android/ReferenceFilesAttribute.cs
@@ -1,7 +1,7 @@
 using System;
 
 namespace Android {
-
+	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
 	public abstract class ReferenceFilesAttribute : Attribute
 	{
 		internal ReferenceFilesAttribute () {}
diff --git a/src/Mono.Android/Java.Interop/JavaLibraryReferenceAttribute.cs b/src/Mono.Android/Java.Interop/JavaLibraryReferenceAttribute.cs
index c0262389629..a859cc23135 100644
--- a/src/Mono.Android/Java.Interop/JavaLibraryReferenceAttribute.cs
+++ b/src/Mono.Android/Java.Interop/JavaLibraryReferenceAttribute.cs
@@ -3,6 +3,7 @@
 namespace Java.Interop {
 
 	[AttributeUsage (AttributeTargets.Assembly, AllowMultiple = true)]
+	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
 	public class JavaLibraryReferenceAttribute : Android.ReferenceFilesAttribute
 	{
 		public JavaLibraryReferenceAttribute (string filename)
diff --git a/src/Xamarin.Android.Build.Tasks/Tasks/GetAdditionalResourcesFromAssemblies.cs b/src/Xamarin.Android.Build.Tasks/Tasks/GetAdditionalResourcesFromAssemblies.cs
index 62b677a30d1..fc8b6cb6916 100644
--- a/src/Xamarin.Android.Build.Tasks/Tasks/GetAdditionalResourcesFromAssemblies.cs
+++ b/src/Xamarin.Android.Build.Tasks/Tasks/GetAdditionalResourcesFromAssemblies.cs
@@ -113,6 +113,9 @@ internal static string ErrorMessage (CustomAttributeValue<object> attributeValue
 		void AddAttributeValue (ICollection<string> items, CustomAttributeValue<object> attributeValue, string errorCode, string errorFmt,
 			bool isDirectory, string fullPath, string attributeFullName)
 		{
+			LogCodedWarning ("XA0121",
+				$"Assembly '{Path.GetFileName (fullPath)}' is using a deprecated attribute '[assembly: {attributeFullName}]'. Use a newer version of this NuGet package or notify the library author.");
+
 			if (attributeValue.NamedArguments.Length == 0 || attributeValue.FixedArguments.Length != 1) {
 				LogCodedWarning (errorCode, "Attribute {0} doesn't have expected one constructor agrument", attributeFullName);
 				return;
diff --git a/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs b/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs
index 2d71c2284c6..c63bad15f45 100644
--- a/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs
+++ b/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs
@@ -2296,6 +2296,7 @@ public void ResourceExtraction ([Values (true, false)] bool useAapt2)
 			using (var builder = CreateApkBuilder (Path.Combine ("temp", TestName))) {
 				Assert.IsTrue (builder.DesignTimeBuild (proj), "design-time build should have succeeded");
 				Assert.IsTrue (builder.Build (proj), "build should have succeeded");
+				Assert.IsTrue (StringAssertEx.ContainsText (builder.LastBuildOutput, "XA0121"), "Output should contain XA0121 warnings");
 				var targetAar = Path.Combine (CachePath, "Xamarin.Android.Support.v7.AppCompat", "23.1.1.0",
 					"content", "m2repository", "com", "android", "support", "appcompat-v7", "23.1.1", "appcompat-v7-23.1.1.aar");
 				if (File.Exists (targetAar)) {