There are over three thousand modules compatible with Drupal 9 already. That is no small feat! However, there is almost another three thousand projects that only need an info.yml file fixed and hundreds more with only a few issues to fix. If you own a drupal.org project of any kind, or are interested to see some projects become compatible, this is your contribution group.
The best way to interactively participate is in the #d9-readiness channel on drupal.org/slack.