The main problem is the fact, that PHP 8.0 removed the context parameter which we used to show the original source of the deprecation.
See context parameter of error handler deprecated · Issue #15195 · cakephp/cakephp · GitHub
If you can’t see the deprecation in the frontend via the debug_kit toolbar then I would only recommend you to grep for that code part like
grep -R 'Router::scope(' .