The core: 8.x key must not be used here.

Hello Maintainers,
I have provided a patch for this issue.
Please review.

Thanks!

Good catch, thanks for the patch. This has now been fixed in the 1.x branch.

Automatically closed - issue fixed for 2 weeks with no activity.

Modules, themes. and profiles are allowed to use the following lines in their .info.yml file.

core: 8.x
core_version_requirement: ^8 || ^9

That is described in New 'core_version_requirement' key in info.yml files for modules, themes and profiles → allows Composer semantic version constraints including specifying multiple major versions of core, which states:

For example a module that is compatible with Drupal 8 and Drupal 9 can have a info.yml file like this

name: My Module
type: module
core: 8.x
core_version_requirement: ^8 || ^9