With seven days to go until Drupal 8's end of life (on November 2, 2021), now is a good time to take stock of your Drupal 8 sites' modules. Use Upgrade Status to check for environment and module compatibility with Drupal 9.
Various people in the Upgrade Status issue queue are concerned about the module requiring Composer though. While Drupal 8 or 9 do not require Composer to work, getting any extension installed on your site that depends on third party components is a sizeable challenge without Composer. With Drupal 8 core dependent on various third party components, it was inevitable that a dependency manager will be needed to build Drupal sites. Upgrade Status itself does require various third party components to check for deprecated API uses and even to locate your Drupal site root. So you cannot avoid using Composer to even check for Drupal 9 compatibility.