Add a deprecation comment.

plugins/terminalapp/terminalapp.plugin.zsh

@@ -0,0 +1,6 @@
+# This file is intentionally empty.
+#
+# The terminalapp plugin is deprecated and may be removed in a future release.
+# Its functionality has been folded in to the core lib/termsupport.zsh, which
+# is loaded for all users. You can remove terminalapp from your $plugins list
+# once all your systems are updated to the current version of Oh My Zsh.