Add missing deprecation warning left out of !2842

lib/pleroma/config/deprecation_warnings.ex

@@ -41,7 +41,8 @@ defmodule Pleroma.Config.DeprecationWarnings do
          :ok <- check_gun_pool_options(),
          :ok <- check_activity_expiration_config(),
          :ok <- check_remote_ip_plug_name(),
-         :ok <- check_uploders_s3_public_endpoint() do
+         :ok <- check_uploders_s3_public_endpoint(),
+         :ok <- check_old_chat_shoutbox() do
       :ok
     else
       _ ->
@@ -215,4 +216,27 @@ defmodule Pleroma.Config.DeprecationWarnings do
       :ok
     end
   end
+
+  @spec check_old_chat_shoutbox() :: :ok | nil
+  def check_old_chat_shoutbox do
+    instance_config = Pleroma.Config.get([:instance])
+    chat_config = Pleroma.Config.get([:chat]) || []
+
+    use_old_config =
+      Keyword.has_key?(instance_config, :chat_limit) or
+        Keyword.has_key?(chat_config, :enabled)
+
+    if use_old_config do
+      Logger.error("""
+      !!!DEPRECATION WARNING!!!
+      Your config is using the old namespace for the Shoutbox configuration. You need to convert to the new namespace. e.g.,
+      \n* `config :pleroma, :chat, enabled` and `config :pleroma, :instance, chat_limit` are now equal to:
+      \n* `config :pleroma, :shout, enabled` and `config :pleroma, :shout, limit`
+      """)
+
+      :error
+    else
+      :ok
+    end
+  end
 end