We discussed that in Portland but I noted that we do not even have an issue for it yet:
- It's confusing that we now have mostly everything below \Drupal\Core\Entity but some stuff remains in the module
- Plugins can be implemented in Core components now, so that's not a cause for having the module anymore.
- Do we still need a module as CMI-config owner?
So even if CMI config remains in a module, imo the main API, i.e. the interfaces you type-hint on should be all in the Core component.