Missing proxy class 'Drupal\book_tree_menu\ProxyClass\oscBookManager' for lazy service 'book.manager

Yes, this is needed, but in the patch provided you had the proxy class extend from BookManager, but I'm not sure why. I don't think you need to overwrite anything at all from what the script created. I created a Merge Request with the unaltered code of the created proxy class.

Also bumping to major as this is needed for Drupal 10.2.x.

Creating a patch to add the file generated by the script.

Patch #6 worked for me - the warning "Missing proxy class 'Drupal\book_tree_menu\ProxyClass\oscBookManager' for lazy service 'book.manager'" was previously appearing after every drush cr, it's gone after applying the patch.