How to get node Bundle in hook_form_alter on field config form?
You're looking for FieldDefinitionInterface::getTargetBundle


  Gets the bundle the field is attached to.
  
  This method should not be confused with EntityInterface::bundle() (configurable fields are config entities, and thus implement both interfaces):
  
  
  FieldDefinitionInterface::getTargetBundle() answers "as a field, which bundle are you attached to?".
  EntityInterface::bundle() answers "as a (config) entity, what is your own bundle?" (not relevant in our case, the config entity types used to store the definitions of configurable fields do not have bundles).
  


e.g. 

$target_bundle = $field-&gt;getTargetBundle();