What are the actions that a plugin can hook on to for processing when the user deletes a category?
It has number of hooks, the one that comes right before delete is:
do_action( 'delete_term_taxonomy', $tt_id );
Hooks after delete:
do_action( 'deleted_term_taxonomy', $tt_id ); do_action('delete_term', $term, $tt_id, $taxonomy); do_action("delete_$taxonomy", $term, $tt_id);
There are also few earlier hooks that deal with remapping children terms.