WordPress at a glance
action is not described

wp_uninitialize_site action-hook . WP 5.1.0

Fires when a site's uninitialization routine should be executed.

Usage

add_action( 'wp_uninitialize_site', 'action_function_name_9554' );
function action_function_name_9554( $old_site ){
	// action...
}
$old_site(WP_Site)
Deleted site object.

Where the hook is called

wp_delete_site()
wp_uninitialize_site
wp-includes/ms-site.php 252
do_action( 'wp_uninitialize_site', $old_site );

Where the hook is used (in WP core)

wp-includes/ms-default-filters.php 53
add_action( 'wp_uninitialize_site', 'wp_uninitialize_site', 10, 1 );