Upgrade Status is growing to be an almost (if not entirely) indispensable tool for Drupal 8 sites to asses and prepare for their upgrade to Drupal 9. With the latest additions of the Drupal core version and environment checklist as well as additional deprecation checking for Twig, deprecated libraries, deprecated theme functions, info.yml files and composer.json files, the results are much more complete. Unfortunately not all of those are possible in drupal-check due to Drupal not being bootstrapped there. With the addition of two drush commands though, this expanded set of results is available for the command line as well, including for CI system integration.
A relatively new development is Palantir's drupal-rector which is amazing at providing automated fixes for an increasing number of deprecated APIs. I also built a UI on top of that called Upgrade Rector to make it easier to propose fixes for your team's custom modules as well as your contributed modules. You should always keep the Drupal 9 plan of projects in mind, and contributed in accordance with their indicated Drupal 9 plan, which is well facilitated if you use Upgrade Rector within the Upgrade Status UI. Check out this demo video where I walk through the latest improvements:
I hope you find the tools useful and continue to contribute to both projects to help make the upgrade path to Drupal 9 even easier.