Deprecate the onStdin handler on the generic IFuture.

packages/services/src/kernel/future.ts

@@ -57,6 +57,17 @@ export abstract class KernelFutureHandler<
     return this._done.promise;
   }
 
+  /**
+   * Get the stdin handler.
+   * @deprecated - This method exists to satisfy the `IFuture` interface and
+   * will be removed in a future version.
+   */
+  get onStdin(): (
+    msg: KernelMessage.IStdinMessage
+  ) => void | PromiseLike<void> {
+    return undefined;
+  }
+
   /**
    * Get the reply handler.
    */

packages/services/src/kernel/kernel.ts

@@ -763,6 +763,17 @@ export namespace Kernel {
       | KernelMessage.IShellMessage
       | KernelMessage.IControlMessage = KernelMessage.IShellMessage
   > extends IDisposable {
+    /**
+     * The stdin handler for the kernel future.
+     * @deprecated - This handler properly belongs in `IShellFuture` and will be
+     * removed in a future version of `@jupyterlab/services`.
+     *
+     * #### Notes
+     * If the handler returns a promise, all kernel message processing pauses
+     * until the promise is resolved.
+     */
+    onStdin: (msg: KernelMessage.IStdinMessage) => void | PromiseLike<void>;
+
     /**
      * The original outgoing message.
      */