Missing requirement in composer.json should not generate a warning

The patch looks right, although the revised error message is a little long:

-          'message' => "The drupal/core requirement is not Drupal 9 compatible. Having a composer.json is not a requirement for Drupal 9 compatibility but if there is one, it should include a drupal/core requirement compatible with Drupal 9.",
+          'message' => "The drupal/core requirement is not Drupal 9 compatible. Having a composer.json is not a requirement for Drupal 9 compatibility. Even if there is a composer.json file, it is not necessary to have a drupal/core requirement defined in it. But if there is a composer.json file and the drupal/core requirement is defined in it, it should be defined as compatible with Drupal 9.",

Maybe something more prescriptive would be better:

The drupal/core requirement is not Drupal 9 compatible. Either remove it or update it to be compatible with Drupal 9. Having a composer.json is not a requirement for Drupal 9 compatibility.

Consider removing the last sentence: I do not think we want to encourage module maintainers to remove composer.json.

I tried to test locally, but now I get

Server error: `POST https://drupal.lndo.site/admin/reports/upgrade-status/analyze/module/typ...` resulted in a `500 500 Service unavailable (with message)` response: The website encountered an unexpected error. Please try again later.

Any idea where that comes from? I did update Lando and rebuild my containers since the earlier test.

I think the standard documentation about composer.json is https://www.drupal.org/docs/8/creating-custom-modules/add-a-composerjson.... You might want to add a section there about the best practices for D9 compatibility.

Related: https://www.drupal.org/docs/8/creating-custom-modules/let-drupal-8-know-.... This already has good information on core_version_requirement and core: 8.x.