WordPress at a glance
filter is not described

delete_(meta_type)_metadata filter-hook . WP 3.1.0

Filters whether to delete metadata of a specific type.

The dynamic portion of the hook, $meta_type, refers to the meta object type (comment, post, term, or user). Returning a non-null value will effectively short-circuit the function.

Usage

add_filter( 'delete_(meta_type)_metadata', 'filter_function_name_9926', 10, 5 );
function filter_function_name_9926( $delete, $object_id, $meta_key, $meta_value, $delete_all ){
	// filter...

	return $delete;
}
$delete(null/true/false)
Whether to allow metadata deletion of the given type.
$object_id(int)
Object ID.
$meta_key(string)
Meta key.
$meta_value(mixed)
Meta value. Must be serializable if non-scalar.
$delete_all(true/false)
Whether to delete the matching metadata entries for all objects, ignoring the specified $object_id.
Default: false

Where the hook is called

delete_metadata()
delete_(meta_type)_metadata
wp-includes/meta.php 351
$check = apply_filters( "delete_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $delete_all );

Where the hook is used (in WP core)

wp-includes/default-filters.php 104
add_filter( 'delete_term_metadata', 'wp_check_term_meta_support_prefilter' );