allout-structure-deleted-hook is a variable defined in `allout.el'.
Its value is

Abnormal hook run after deleting subtrees from an Allout outline.
Functions on the hook must take two arguments:

- DEPTH -- integer indicating the depth of the subtree that was deleted.
- REMOVED-FROM -- integer indicating the point where the subtree was removed.

Some edits that remove or invalidate items may be missed by this hook:
specifically edits that native allout routines do not control.

This hook might be invoked multiple times by a single command.

You can customize this variable.