diff --git a/lib/externallib.php b/lib/externallib.php
index cd4579b8985..01b427d0065 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -24,6 +24,10 @@
 
 use core_external\util;
 
+// Please note that this file and all of the classes and functions listed below will be deprecated from Moodle 4.6.
+// This deprecation is delayed to aid plugin developers when maintaining plugins for multiple Moodle versions.
+// See MDL-76583 for further information.
+
 class_alias(\core_external\external_api::class, 'external_api');
 class_alias(\core_external\restricted_context_exception::class, 'restricted_context_exception');
 class_alias(\core_external\external_description::class, 'external_description');
diff --git a/lib/upgrade.txt b/lib/upgrade.txt
index 60eda59ba28..c48ff953a0e 100644
--- a/lib/upgrade.txt
+++ b/lib/upgrade.txt
@@ -25,6 +25,31 @@ information provided here is intended especially for developers.
   initials bar without the bootstrapping and form handling on each initials bar. If you use this mini render,
   you'll need to implement your own form handling. Example usage can be found within the grader report.
 * There is a new helper function mtrace_exception to help with reporting exceptions you have caught in scheduled tasks.
+* The following parts of the external API have been moved to the core_external subsystem:
+  Classes:
+    - external_api                      => core_external\external_api
+    - external_description              => core_external\external_description
+    - external_files                    => core_external\files
+    - external_format_value             => core_external\external_format_value
+    - external_function_parameters      => core_external\external_function_parameters
+    - external_multiple_structure       => core_external\external_multiple_structure
+    - external_settings                 => core_external\external_settings
+    - external_single_structure         => core_external\external_single_structure
+    - external_util                     => core_external\util
+    - external_value                    => core_external\external_value
+    - external_warnings                 => core_external\external_warnings
+    - restricted_context_exception      => core_external\restricted_context_exception
+  Functions:
+    - external_format_string()          => core_external\util::format_string()
+    - external_format_text()            => core_external\util::format_text()
+    - external_create_service_token()   => core_external\util::generate_token()
+    - external_generate_token()         => core_external\util::generate_token()
+    - external_generate_token_for_current_user()
+                                        => core_external\util::generate_token_for_current_user()
+    - external_log_token_request        => core_external\util::log_token_request()
+
+  The old class locations have been aliased for backwards compatibility and will emit a deprecation notice in a future
+  release.
 
 === 4.1 ===