diff --git a/src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs b/src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs
index 910193c4ed0d5..3221043e6b5ea 100644
--- a/src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs
+++ b/src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs
@@ -71,6 +71,8 @@ public Process() { }
         public long PrivateMemorySize64 { get { throw null; } }
         public System.TimeSpan PrivilegedProcessorTime { get { throw null; } }
         public string ProcessName { get { throw null; } }
+        [System.Runtime.Versioning.SupportedOSPlatform("windows")]
+        [System.Runtime.Versioning.SupportedOSPlatform("linux")]
         public System.IntPtr ProcessorAffinity { get { throw null; } set { } }
         public bool Responding { get { throw null; } }
         public Microsoft.Win32.SafeHandles.SafeProcessHandle SafeHandle { get { throw null; } }
@@ -237,11 +239,13 @@ internal ProcessThread() { }
         public int Id { get { throw null; } }
         public int IdealProcessor { set { } }
         public bool PriorityBoostEnabled { get { throw null; } set { } }
-        public System.Diagnostics.ThreadPriorityLevel PriorityLevel { get { throw null; } [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")] set { } }
+        public System.Diagnostics.ThreadPriorityLevel PriorityLevel { [System.Runtime.Versioning.SupportedOSPlatform("windows")] [System.Runtime.Versioning.SupportedOSPlatform("linux")] [System.Runtime.Versioning.SupportedOSPlatform("freebsd")] get { throw null; } [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")] set { } }
         public System.TimeSpan PrivilegedProcessorTime { get { throw null; } }
         [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")]
         public System.IntPtr ProcessorAffinity { set { } }
         public System.IntPtr StartAddress { get { throw null; } }
+        [System.Runtime.Versioning.SupportedOSPlatform("windows")]
+        [System.Runtime.Versioning.SupportedOSPlatform("linux")]
         public System.DateTime StartTime { get { throw null; } }
         public System.Diagnostics.ThreadState ThreadState { get { throw null; } }
         public System.TimeSpan TotalProcessorTime { get { throw null; } }
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs
index f0eb46b0f67e0..58d297728f5ca 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs
@@ -526,6 +526,8 @@ public string ProcessName
         ///       or sets which processors the threads in this process can be scheduled to run on.
         ///    </para>
         /// </devdoc>
+        [SupportedOSPlatform("windows")]
+        [SupportedOSPlatform("linux")]
         public IntPtr ProcessorAffinity
         {
             get
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessManager.Unix.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessManager.Unix.cs
index a4b1e171768af..5df7ece329e1d 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessManager.Unix.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessManager.Unix.cs
@@ -72,9 +72,5 @@ internal static void ThrowIfRemoteMachine(string machineName)
                 throw new PlatformNotSupportedException(SR.RemoteMachinesNotSupported);
             }
         }
-        public static IntPtr GetMainWindowHandle(int processId)
-        {
-            throw new PlatformNotSupportedException(); // Window handle is a Windows concept
-        }
     }
 }
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 cf098ac0ef0ce..a6b17bee36fb5 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
@@ -23,13 +23,9 @@ private ThreadPriorityLevel PriorityLevelCore
             }
         }
 
-        /// <summary>Returns the time the associated thread was started.</summary>
-        public DateTime StartTime
-        {
-            // kinfo_proc has one  entry per thread but ki_start seems to be same for
-            // all threads e.g. reflects process start. This may be re-visited later.
-            get { throw new PlatformNotSupportedException(); }
-        }
+        // kinfo_proc has one  entry per thread but ki_start seems to be same for
+        // all threads e.g. reflects process start. This may be re-visited later.
+        private DateTime GetStartTime() => throw new PlatformNotSupportedException();
 
         /// <summary>
         /// Returns the amount of time the associated thread has spent utilizing the CPU.
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Linux.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Linux.cs
index 3a0f2342b3544..853c06dcdfa2c 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Linux.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Linux.cs
@@ -41,11 +41,7 @@ public TimeSpan PrivilegedProcessorTime
             }
         }
 
-        /// <summary>Returns the time the associated thread was started.</summary>
-        public DateTime StartTime
-        {
-            get { return Process.BootTimeToDateTime(Process.TicksToTimeSpan(GetStat().starttime)); }
-        }
+        private DateTime GetStartTime() => Process.BootTimeToDateTime(Process.TicksToTimeSpan(GetStat().starttime));
 
         /// <summary>
         /// Returns the amount of time the associated thread has spent utilizing the CPU.
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.OSX.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.OSX.cs
index eca275a352e04..e95abff9af741 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.OSX.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.OSX.cs
@@ -25,11 +25,7 @@ private ThreadPriorityLevel PriorityLevelCore
         /// </summary>
         public TimeSpan PrivilegedProcessorTime => new TimeSpan((long)GetThreadInfo().pth_system_time);
 
-        /// <summary>Returns the time the associated thread was started.</summary>
-        public DateTime StartTime
-        {
-            get { throw new PlatformNotSupportedException(); } // macOS does not provide a way to get this data
-        }
+        private DateTime GetStartTime() => throw new PlatformNotSupportedException(); // macOS does not provide a way to get this data
 
         /// <summary>
         /// Returns the amount of time the associated thread has spent using the CPU.
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.UnknownUnix.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.UnknownUnix.cs
index 8754ffdf5e531..6d947625ddd6b 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.UnknownUnix.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.UnknownUnix.cs
@@ -25,11 +25,7 @@ public TimeSpan PrivilegedProcessorTime
             get { throw new PlatformNotSupportedException(); }  // Not available on POSIX
         }
 
-        /// <summary>Returns the time the associated thread was started.</summary>
-        public DateTime StartTime
-        {
-            get { throw new PlatformNotSupportedException(); }  // Not available on POSIX
-        }
+        private DateTime GetStartTime() => throw new PlatformNotSupportedException(); // Not available on POSIX
 
         /// <summary>
         /// Returns the amount of time the associated thread has spent utilizing the CPU.
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Windows.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Windows.cs
index f46fd604e5e21..85c96684c2bc7 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Windows.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.Windows.cs
@@ -124,11 +124,7 @@ public TimeSpan PrivilegedProcessorTime
             get { return GetThreadTimes().PrivilegedProcessorTime; }
         }
 
-        /// <summary>Returns the time the associated thread was started.</summary>
-        public DateTime StartTime
-        {
-            get { return GetThreadTimes().StartTime; }
-        }
+        private DateTime GetStartTime() => GetThreadTimes().StartTime;
 
         /// <summary>
         /// Returns the amount of time the associated thread has spent utilizing the CPU.
diff --git a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.cs b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.cs
index e147573bc13bc..215e67e8928d5 100644
--- a/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.cs
+++ b/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.cs
@@ -88,6 +88,9 @@ public bool PriorityBoostEnabled
         /// </devdoc>
         public ThreadPriorityLevel PriorityLevel
         {
+            [SupportedOSPlatform("windows")]
+            [SupportedOSPlatform("linux")]
+            [SupportedOSPlatform("freebsd")]
             get
             {
                 if (!_priorityLevel.HasValue)
@@ -136,6 +139,14 @@ public ThreadWaitReason WaitReason
             }
         }
 
+        /// <summary>Returns the time the associated thread was started.</summary>
+        [SupportedOSPlatform("windows")]
+        [SupportedOSPlatform("linux")]
+        public DateTime StartTime
+        {
+            get => GetStartTime();
+        }
+
         /// <devdoc>
         ///     Helper to check preconditions for property access.
         /// </devdoc>