This setting is only used by the shrink action.

action: shrink
description: >-
  Shrink selected indices on the node with the most available space.
  Delete source index after successful shrink.
  shrink_node: DETERMINISTIC
  delete_after: True
  - filtertype: ...

The default value of this setting is True. After an index has been successfully shrunk, the source index will be deleted or preserved based on the value of this setting.