diff --git a/src/Runner.Common/Constants.cs b/src/Runner.Common/Constants.cs
index 05b939270..9ad29b4ac 100644
--- a/src/Runner.Common/Constants.cs
+++ b/src/Runner.Common/Constants.cs
@@ -128,7 +128,7 @@ namespace GitHub.Runner.Common
                     public static readonly string Ephemeral = "ephemeral";
                     public static readonly string Help = "help";
                     public static readonly string Replace = "replace";
-                    public static readonly string Once = "once"; // TODO: Remove in 10/2021
+                    public static readonly string Once = "once"; // Keep this around since customers still relies on it
                     public static readonly string RunAsService = "runasservice";
                     public static readonly string Unattended = "unattended";
                     public static readonly string Version = "version";
diff --git a/src/Runner.Listener/CommandSettings.cs b/src/Runner.Listener/CommandSettings.cs
index b5e1c1a69..da0d301fb 100644
--- a/src/Runner.Listener/CommandSettings.cs
+++ b/src/Runner.Listener/CommandSettings.cs
@@ -31,6 +31,7 @@ namespace GitHub.Runner.Listener
             Constants.Runner.CommandLine.Flags.Commit,
             Constants.Runner.CommandLine.Flags.Ephemeral,
             Constants.Runner.CommandLine.Flags.Help,
+            Constants.Runner.CommandLine.Flags.Once,
             Constants.Runner.CommandLine.Flags.Replace,
             Constants.Runner.CommandLine.Flags.RunAsService,
             Constants.Runner.CommandLine.Flags.Unattended,
@@ -68,7 +69,7 @@ namespace GitHub.Runner.Listener
         public bool Version => TestFlag(Constants.Runner.CommandLine.Flags.Version);
         public bool Ephemeral => TestFlag(Constants.Runner.CommandLine.Flags.Ephemeral);
 
-        // TODO: Remove in 10/2021
+        // Keep this around since customers still relies on it
         public bool RunOnce => TestFlag(Constants.Runner.CommandLine.Flags.Once);
 
         // Constructor.
diff --git a/src/Runner.Listener/Runner.cs b/src/Runner.Listener/Runner.cs
index 8f55cd348..bdeff538d 100644
--- a/src/Runner.Listener/Runner.cs
+++ b/src/Runner.Listener/Runner.cs
@@ -233,8 +233,14 @@ namespace GitHub.Runner.Listener
                     Trace.Info($"Set runner startup type - {startType}");
                     HostContext.StartupType = startType;
 
+                    if (command.RunOnce)
+                    {
+                        _term.WriteLine("Warning: '--once' is going to be deprecated in the future, please consider using '--ephemeral' during runner registration.", ConsoleColor.Yellow);
+                        _term.WriteLine("https://docs.github.com/en/actions/hosting-your-own-runners/autoscaling-with-self-hosted-runners#using-ephemeral-runners-for-autoscaling", ConsoleColor.Yellow);
+                    }
+
                     // Run the runner interactively or as service
-                    return await RunAsync(settings, command.RunOnce || settings.Ephemeral);  // TODO: Remove RunOnce later.
+                    return await RunAsync(settings, command.RunOnce || settings.Ephemeral);
                 }
                 else
                 {