diff --git a/lib/classes/component.php b/lib/classes/component.php
index 333ff0f0e4b..e7f3eb4b324 100644
--- a/lib/classes/component.php
+++ b/lib/classes/component.php
@@ -383,6 +383,24 @@ class core_component {
             return false;
         }
 
+        // Check for key classes which block access to the upgrade in some way.
+        // Note: This list should be kept _extremely_ minimal and generally
+        // when adding a newly discovered classes older ones should be removed.
+        // Always keep moodle_exception in place.
+        $keyclasses = [
+            \core\exception\moodle_exception::class,
+            \core\output\bootstrap_renderer::class,
+        ];
+        foreach ($keyclasses as $classname) {
+            if (!array_key_exists($classname, $cache['classmap'])) {
+                // The cache is missing some key classes. This is likely before the upgrade has run.
+                error_log(
+                    "The '{$classname}' class was not found in the component class cache. Resetting the classmap.",
+                );
+                return false;
+            }
+        }
+
         return true;
     }
 
diff --git a/lib/setup.php b/lib/setup.php
index c0cf3891d1c..9c3fb46124e 100644
--- a/lib/setup.php
+++ b/lib/setup.php
@@ -620,10 +620,6 @@ if (defined('ABORT_AFTER_CONFIG')) {
 
 require_once($CFG->libdir .'/setuplib.php');        // Functions that MUST be loaded first.
 
-// TODO MDL-81933 Remove after Moodle 4.5 release.
-require_once($CFG->libdir . '/classes/exception/moodle_exception.php');       // Required by some other legacy libraries.
-require_once($CFG->libdir . '/classes/output/bootstrap_renderer.php');        // Required by some other legacy libraries.
-
 // Load up standard libraries.
 require_once($CFG->libdir .'/filterlib.php');       // Functions for filtering test as it is output.
 require_once($CFG->libdir .'/ajax/ajaxlib.php');    // Functions for managing our use of JavaScript and YUI.