Display hidden WordPress plugin and theme update notifications on the update-core screen.

functions.php

<?php

/**
 * Debug Pending Updates
 *
 * Displays hidden plugin and theme updates on update-core screen.
 * Source: https://stackoverflow.com/a/52132227
 */
function debug_pending_updates() {

  // Rough safety nets
  if ( ! is_user_logged_in() || ! current_user_can( 'update_plugins' ) || ! current_user_can( 'update_themes' ) ) return;

  $output = "";

  // Check plugins
  $plugin_updates = get_site_transient( 'update_plugins' );
  if ( $plugin_updates && ! empty( $plugin_updates->response ) ) {
    foreach ( $plugin_updates->response as $plugin => $details ) {
      $output .= "<p><strong>Plugin</strong> <u>$plugin</u> is reporting an available update.</p>";
    }
  }

  // Check themes
  wp_update_themes();
  $theme_updates = get_site_transient( 'update_themes' );
  if ( $theme_updates && ! empty( $theme_updates->response ) ) {
    foreach ( $theme_updates->response as $theme => $details ) {
      $output .= "<p><strong>Theme</strong> <u>$theme</u> is reporting an available update.</p>";
    }
  }

  if ( empty( $output ) ) $output = "No pending updates found in the database.";

  echo "<h2>Pending updates</h2>" . $output;
}
add_action( 'core_upgrade_preamble', 'debug_pending_updates' );