We are glad to announce that a new version of Search404 2.0.0 was released on the same day Drupal 9 was released. This version fully supports Drupal 9.
In this release of the module, we’ve chosen to go with the fully semantic versioning model, which means, version 2.0.0 supports Drupal Core 8.8.3 or a higher version. The 8.x-1.0 release will be there for some more time to support older versions. The 2.0.0 release mainly removes the deprecated code and uses the new solution that is supported by Drupal 9.