More recently I worked on making Upgrade Status work meaningfully on Drupal 9 with Andrey Postnikov. Released Upgrade Status 8.x-3.5-alpha1 today to let you test this out. (You may need to use
composer require --dev phpspec/prophecy-phpunit to make your phpunit setup complete).
How is this different from prior releases of Upgrade Status? It should run very similar on Drupal 8 as it did before. However prior releases of Upgrade Status explicitly forbid running it on Drupal 9 as the UI was very focused on the transition from 8 to 9. Now the UI elements are adapted and in some cases more general to support running either on Drupal 8 or 9.