With six 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.
While a lot of Drupal extensions are now compatible with Drupal 9, it can happen that you encounter an incompatible extension. In case adopting the extension fits into your plans, that is the best way forward. However, there may be some delay until you can adopt the project and it may not fit into your plans to adopt in the first place. There are a few solutions to use drupal.org projects that are distributed as incompatible on your Drupal 9 sites.