Follow up for #1971384: [META] Convert page callbacks to controllers and #1913618: Change notice: Convert EntityFormControllerInterface to extend FormInterface
Whilst we have _entity_form for our defaults in routing, it can't be expected to deal with entity add callbacks, as these need to create a psuedo entity first and the keys they need to set vary (eg menu items may need a plid) or are dynamic (eg node/add/type and block/add/custom_block_type).
This leaves the controller methods for callbacks like block/add/custom_block_type #1978166: Convert block/add/{%custom_block_type} to Controller needing to call entity_get_form from their controllers which breaks our OO goal.
This issue tracks moving entity_get_form to Drupal\Core\Entity\EntityManager::getForm()
