diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.FreeBSD.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.FreeBSD.cs
index 3fd30ba9ba2c50..92aaf181c6f90a 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.FreeBSD.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.FreeBSD.cs
@@ -50,6 +50,9 @@ public TimeSpan TotalProcessorTime
         /// Returns the amount of time the associated thread has spent running code
         /// inside the application (not the operating system core).
         /// </summary>
+        [UnsupportedOSPlatform("ios")]
+        [UnsupportedOSPlatform("tvos")]
+        [SupportedOSPlatform("maccatalyst")]
         public TimeSpan UserProcessorTime
         {
             get
diff --git a/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.cs b/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.cs
index 53ae907c7bfe7f..02f84782fbcb6b 100644
--- a/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.cs
+++ b/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.AnyMobile.cs
@@ -617,6 +617,8 @@ public bool UseProxy
         }
 
         [UnsupportedOSPlatform("browser")]
+        [UnsupportedOSPlatform("ios")]
+        [UnsupportedOSPlatform("tvos")]
         public IWebProxy? Proxy
         {
             get
diff --git a/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs b/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs
index be30ca79090c95..1caf3d819be853 100644
--- a/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs
+++ b/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs
@@ -1205,7 +1205,9 @@ internal void HandleAltSvc(IEnumerable<string> altSvcHeaderValues, TimeSpan? res
 
                 if (!nextAuthorityPersist)
                 {
+#if !ILLUMOS && !SOLARIS
                     _poolManager.StartMonitoringNetworkChanges();
+#endif
                 }
             }
         }
diff --git a/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs b/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs
index f118d151c692ae..fdfcd5fa7cce01 100644
--- a/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs
+++ b/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs
@@ -44,7 +44,9 @@ internal sealed class HttpConnectionPoolManager : IDisposable
         private readonly IWebProxy? _proxy;
         private readonly ICredentials? _proxyCredentials;
 
+#if !ILLUMOS && !SOLARIS
         private NetworkChangeCleanup? _networkChangeCleanup;
+#endif
 
         /// <summary>
         /// Keeps track of whether or not the cleanup timer is running. It helps us avoid the expensive
@@ -150,6 +152,7 @@ public HttpConnectionPoolManager(HttpConnectionSettings settings)
             }
         }
 
+#if !ILLUMOS && !SOLARIS
         /// <summary>
         /// Starts monitoring for network changes. Upon a change, <see cref="HttpConnectionPool.OnNetworkChanged"/> will be
         /// called for every <see cref="HttpConnectionPool"/> in the <see cref="HttpConnectionPoolManager"/>.
@@ -219,6 +222,7 @@ public void Dispose()
                 GC.SuppressFinalize(this);
             }
         }
+#endif
 
         public HttpConnectionSettings Settings => _settings;
         public ICredentials? ProxyCredentials => _proxyCredentials;
@@ -454,7 +458,9 @@ public void Dispose()
                 pool.Value.Dispose();
             }
 
+#if !ILLUMOS && !SOLARIS
             _networkChangeCleanup?.Dispose();
+#endif
         }
 
         /// <summary>Sets <see cref="_cleaningTimer"/> and <see cref="_timerIsRunning"/> based on the specified timeout.</summary>
diff --git a/src/libraries/System.Net.NetworkInformation/ref/System.Net.NetworkInformation.cs b/src/libraries/System.Net.NetworkInformation/ref/System.Net.NetworkInformation.cs
index 7e5c95ee00f7bb..df04b533cca1b0 100644
--- a/src/libraries/System.Net.NetworkInformation/ref/System.Net.NetworkInformation.cs
+++ b/src/libraries/System.Net.NetworkInformation/ref/System.Net.NetworkInformation.cs
@@ -335,7 +335,11 @@ public partial class NetworkChange
         [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
         [System.ObsoleteAttribute("This API supports the .NET Framework infrastructure and is not intended to be used directly from your code.", true)]
         public NetworkChange() { }
+        [System.Runtime.Versioning.UnsupportedOSPlatform("illumos")]
+        [System.Runtime.Versioning.UnsupportedOSPlatform("solaris")]
         public static event System.Net.NetworkInformation.NetworkAddressChangedEventHandler? NetworkAddressChanged { add { } remove { } }
+        [System.Runtime.Versioning.UnsupportedOSPlatform("illumos")]
+        [System.Runtime.Versioning.UnsupportedOSPlatform("solaris")]
         public static event System.Net.NetworkInformation.NetworkAvailabilityChangedEventHandler? NetworkAvailabilityChanged { add { } remove { } }
         [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
         [System.ObsoleteAttribute("This API supports the .NET Framework infrastructure and is not intended to be used directly from your code.", true)]
diff --git a/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.OSX.cs b/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.OSX.cs
index b7c14cad78daea..fbdd96ab0a0388 100644
--- a/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.OSX.cs
+++ b/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.OSX.cs
@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 using Microsoft.Win32.SafeHandles;
 using System.Diagnostics;
 using System.Threading;
@@ -38,6 +39,8 @@ public partial class NetworkChange
         private static readonly AutoResetEvent s_runLoopStartedEvent = new AutoResetEvent(false);
         private static readonly AutoResetEvent s_runLoopEndedEvent = new AutoResetEvent(false);
 
+        [UnsupportedOSPlatform("illumos")]
+        [UnsupportedOSPlatform("solaris")]
         public static event NetworkAddressChangedEventHandler? NetworkAddressChanged
         {
             add
@@ -75,6 +78,8 @@ public static event NetworkAddressChangedEventHandler? NetworkAddressChanged
             }
         }
 
+        [UnsupportedOSPlatform("illumos")]
+        [UnsupportedOSPlatform("solaris")]
         public static event NetworkAvailabilityChangedEventHandler? NetworkAvailabilityChanged
         {
             add
diff --git a/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Unix.cs b/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Unix.cs
index b52087bfda8ea6..1cec0c010bca68 100644
--- a/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Unix.cs
+++ b/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Unix.cs
@@ -6,6 +6,7 @@
 using System.Diagnostics;
 using System.Net.Sockets;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -43,6 +44,8 @@ private static Socket? Socket
         private static Timer? s_availabilityTimer;
         private static bool s_availabilityHasChanged;
 
+        [UnsupportedOSPlatform("illumos")]
+        [UnsupportedOSPlatform("solaris")]
         public static event NetworkAddressChangedEventHandler? NetworkAddressChanged
         {
             add
@@ -83,6 +86,8 @@ public static event NetworkAddressChangedEventHandler? NetworkAddressChanged
             }
         }
 
+        [UnsupportedOSPlatform("illumos")]
+        [UnsupportedOSPlatform("solaris")]
         public static event NetworkAvailabilityChangedEventHandler? NetworkAvailabilityChanged
         {
             add
diff --git a/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Windows.cs b/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Windows.cs
index 745b8492f87e15..dc5dc82ad79d9a 100644
--- a/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Windows.cs
+++ b/src/libraries/System.Net.NetworkInformation/src/System/Net/NetworkInformation/NetworkAddressChange.Windows.cs
@@ -5,6 +5,7 @@
 
 using System.Collections.Generic;
 using System.Net.Sockets;
+using System.Runtime.Versioning;
 using System.Threading;
 
 namespace System.Net.NetworkInformation
@@ -13,6 +14,8 @@ public partial class NetworkChange
     {
         private static readonly object s_globalLock = new object();
 
+        [UnsupportedOSPlatform("illumos")]
+        [UnsupportedOSPlatform("solaris")]
         public static event NetworkAvailabilityChangedEventHandler? NetworkAvailabilityChanged
         {
             add
@@ -25,6 +28,8 @@ public static event NetworkAvailabilityChangedEventHandler? NetworkAvailabilityC
             }
         }
 
+        [UnsupportedOSPlatform("illumos")]
+        [UnsupportedOSPlatform("solaris")]
         public static event NetworkAddressChangedEventHandler? NetworkAddressChanged
         {
             add