Trouble after updating to 2.0
grasmash/composerize-drupal is used to convert a Drupal site that wasn't created with Composer into a Drupal site managed with Composer.
Once the plugin converted the site, it can be removed. After the plugin is removed, modules and themes must be added with composer require, not downloading the archive containing the module/theme files.
If that is the only plugin that isn't compatible with Composer 2.x, you can remove it, as it's not necessary to keep it installed.
Drupal 8.9 is compatible with Composer 2.x; it should not create issues with that Composer version.