The core_version_requirement key specifies what versions of Drupal that the module is compatible with.
name: Exampletype: modulecore: 8.xcore_version_requirement: ^8 || ^9
The above example says that the module is compatible with Drupal 9 and Drupal 9. Notice that the core key is still compulsory here
The chnage
After Drupal 8.8.0 we will not need the core key anymore.
name: Exampletype: modulecore_version_requirement: ^8 || ^9
In fact if you add it with the core key you will get an error like below
Important
The core_version_requirement cannot be used to any version of Drupal prior to 8.7.7 , because it does not recognize the core_version_requirement key.
core_version_requirement: ^8.7 || ^9
This is not valid because ^8.7 would include versions like 8.7.0 which do not recognize the core_version_requirement key.
Reference