Which are the hooks run before/after when a category’s deletion?

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wp_delete_category() is a small wrapper around wp_delete_term() ( source ).

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.