More recently I worked on making Upgrade Status work meaningfully on Drupal 9 with Andrey Postnikov. Released Upgrade Status 8.x-3.5-alpha1 today to let you test this out. (You may need to use
composer require --dev phpspec/prophecy-phpunit to make your phpunit setup complete).
How is this different from prior releases of Upgrade Status? It should run very similar on Drupal 8 as it did before. However prior releases of Upgrade Status explicitly forbid running it on Drupal 9 as the UI was very focused on the transition from 8 to 9. Now the UI elements are adapted and in some cases more general to support running either on Drupal 8 or 9.
Environment requirements are not yet defined for Drupal 10, so that part of the report is intentionally brief. However, we know Drupal 10 will require PHP 8 and the underlying phpstan-drupal library does not work on PHP 8 yet. As a super-important infrastructure piece, I encourage you to sponsor Matt Glaman to make that happen. Even included this link on the module UI.
Should Drupal 9 modules be compatible with Drupal 10 now? Absolutely not! We don't even know the full extent of Drupal 10's API or the final versions of its dependencies. However we do know a lot of deprecated APIs already, some from as far as back as Drupal 8.2.0, and projects can gradually update following those to close the gap with Drupal 10 now.
So running Upgrade Status on Drupal 9 will tell you about
.info.yml file incompatibility errors and
composer.json incompatibility errors, but I would ignore those for now. The more interesting bits are the already fixable deprecated API uses found that were marked deprecated in Drupal 8 and will be removed in Drupal 10. I installed the top 6 most used modules on a test site, to check the results out and there are plenty you can already help move forward in these modules. Most issues found are related to the legacy deprecated assert methods. It is no problem in Drupal 9 to use them, but those will not be present in Drupal 10 next year. Most have 1-1 replacements, so it would be best to build rector rules for them into drupal-rector and support this transition that way. (Also drupal-rector itself needs to support Drupal 9).
I started to work with the Drupal Association to set up a job to figure out the status of Drupal 10 readiness at this time of the 5000+ Drupal 9 compatible projects. (Similar to the deprecation status data we have about Drupal 8 modules tested for Drupal 9 compatibility). It may sound too early as more Drupal 9 APIs will get deprecated for Drupal 10, the reality is we already know a lot of deprecated APIs including those from Symfony and other dependencies. We can use this early data to help prioritise which deprecated APIs to cover with rectors which would allow us to start posting Project Update Bot issues for those that are Drupal 8 to 10 deprecations and are in unsupported Drupal 8 branches. This would allow us to get a headstart on fixing them, automated as much as possible.
At DrupalCon North America 2021 (less than 6 weeks away!) there will be a dedicated Drupal 10 readiness day where you will be able to learn about the state of the new version and will have a chance to work with the lead maintainers for each area to contribute to Drupal core, the tools and readiness of contributed projects.