The implementation of the template_preprocess_admin_block_content hook is not needed

This code was copied over in #2919346: Don't show menu links that you don't have access permission for, but I am not sure this hook is even necessary. The links in admin_block_content are already being filtered in \Drupal\system\Controller\SystemController::overview() through MenuTreeManipulators:

   $manipulators = [
      ['callable' => 'menu.default_tree_manipulators:checkAccess'],
      ['callable' => 'menu.default_tree_manipulators:generateIndexAndSort'],
    ];

Perhaps you can provide a failing test that shows this code is actually still necessary?

I agree with @idebr that function is not even necessary. Here is the corresponding patch.