Drupal9 Readiness and compatibility - Devel
Add core_version_requirement to designate that the module is compatible with Drupal 9.
See https://www.drupal.org/node/3070687 for the refrence.
D9 issues (other than strictly Drupal php code deprecation issues) still to be resolved:
#3124519: Class 'Doctrine\Common\Util\Debug' not found when running Drupal 9.x
#3127750: Using assertContains() with string haystacks is deprecated and will not be supported in PHPUnit 9