diff --git a/ipykernel/ipkernel.py b/ipykernel/ipkernel.py
index 48efa6cd..3419b924 100644
--- a/ipykernel/ipkernel.py
+++ b/ipykernel/ipkernel.py
@@ -491,7 +491,7 @@ async def run(execution: Execution) -> None:
 
         return reply_content
 
-    def do_complete(self, code, cursor_pos):
+    async def do_complete(self, code, cursor_pos):
         """Handle code completion."""
         if _use_experimental_60_completion and self.use_experimental_completions:
             return self._experimental_do_complete(code, cursor_pos)
@@ -625,7 +625,7 @@ def do_history(
             "history": list(hist),
         }
 
-    def do_shutdown(self, restart):
+    async def do_shutdown(self, restart):
         """Handle kernel shutdown."""
         if self.shell:
             self.shell.exit_now = True
diff --git a/ipykernel/kernelbase.py b/ipykernel/kernelbase.py
index d496e0c9..d1ea9033 100644
--- a/ipykernel/kernelbase.py
+++ b/ipykernel/kernelbase.py
@@ -57,6 +57,14 @@
 from ._version import kernel_protocol_version
 from .iostream import OutStream
 
+_AWAITABLE_MESSAGE = (
+    "For consistency across implementations, it is recommended that `{func_name}`"
+    " either be a coroutine function (`async def`) or return an awaitable object"
+    " (like a Future). It might become a requirement in the future."
+    " Coroutine functions and awaitables have been supported since"
+    " ipykernel 6.0 (2021).",
+)
+
 
 def _accepts_parameters(meth, param_names):
     parameters = inspect.signature(meth).parameters
@@ -742,6 +750,12 @@ async def execute_request(self, socket, ident, parent):
 
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="execute_request"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         # Flush output before sending the reply.
         if sys.stdout is not None:
@@ -802,11 +816,17 @@ async def complete_request(self, socket, ident, parent):
         matches = self.do_complete(code, cursor_pos)
         if inspect.isawaitable(matches):
             matches = await matches
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_complete"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         matches = json_clean(matches)
         self.session.send(socket, "complete_reply", matches, parent, ident)
 
-    def do_complete(self, code, cursor_pos):
+    async def do_complete(self, code, cursor_pos):
         """Override in subclasses to find completions."""
         return {
             "matches": [],
@@ -830,6 +850,12 @@ async def inspect_request(self, socket, ident, parent):
         )
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="inspect_request"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         # Before we send this object over, we scrub it for JSON usage
         reply_content = json_clean(reply_content)
@@ -849,6 +875,12 @@ async def history_request(self, socket, ident, parent):
         reply_content = self.do_history(**content)
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="history_request"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
 
         reply_content = json_clean(reply_content)
         msg = self.session.send(socket, "history_reply", reply_content, parent, ident)
@@ -966,6 +998,12 @@ async def shutdown_request(self, socket, ident, parent):
         content = self.do_shutdown(parent["content"]["restart"])
         if inspect.isawaitable(content):
             content = await content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_shutdown"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
         self.session.send(socket, "shutdown_reply", content, parent, ident=ident)
         # same content, but different msg_id for broadcasting on IOPub
         self._shutdown_message = self.session.msg("shutdown_reply", content, parent)
@@ -974,7 +1012,7 @@ async def shutdown_request(self, socket, ident, parent):
 
         self.stop()
 
-    def do_shutdown(self, restart):
+    async def do_shutdown(self, restart):
         """Override in subclasses to do things when the frontend shuts down the
         kernel.
         """
@@ -990,6 +1028,12 @@ async def is_complete_request(self, socket, ident, parent):
         reply_content = self.do_is_complete(code)
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="do_execute"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
         reply_content = json_clean(reply_content)
         reply_msg = self.session.send(socket, "is_complete_reply", reply_content, parent, ident)
         self.log.debug("%s", reply_msg)
@@ -1006,6 +1050,12 @@ async def debug_request(self, socket, ident, parent):
         reply_content = self.do_debug_request(content)
         if inspect.isawaitable(reply_content):
             reply_content = await reply_content
+        else:
+            warnings.warn(
+                _AWAITABLE_MESSAGE.format(func_name="debug_request"),
+                PendingDeprecationWarning,
+                stacklevel=1,
+            )
         reply_content = json_clean(reply_content)
         reply_msg = self.session.send(socket, "debug_reply", reply_content, parent, ident)
         self.log.debug("%s", reply_msg)