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The definition structures for "configurable fields" ($field / $instance structures in D7) are now regular configuration entities.
As a consequence, CRUD actions on field storages and field instance structures trigger the regular entity CRUD hooks, and the previous, Field API specific hooks have been removed.
For performance reasons, and unless your hook implementation intends to react to CRUD actions on various entity types, it is recommended to use the entity-type-specific variants, targeting the 'field_storage_config' and 'field_config' entity types.
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