WordPress at a glance
function is not described

_wp_check_split_nav_menu_terms() WP 4.3.0

If the term being split is a nav_menu, change associations.

No Hooks.

Return

Nothing (null).

Usage

_wp_check_split_nav_menu_terms( $term_id, $new_term_id, $term_taxonomy_id, $taxonomy );
$term_id(int) (required)
ID of the formerly shared term.
$new_term_id(int) (required)
ID of the new term created for the $term_taxonomy_id.
$term_taxonomy_id(int) (required)
ID for the term_taxonomy row affected by the split.
$taxonomy(string) (required)
Taxonomy for the split term.

Code of wp check split nav menu terms: wp-includes/taxonomy.php WP 5.2.2

<?php
function _wp_check_split_nav_menu_terms( $term_id, $new_term_id, $term_taxonomy_id, $taxonomy ) {
	if ( 'nav_menu' !== $taxonomy ) {
		return;
	}

	// Update menu locations.
	$locations = get_nav_menu_locations();
	foreach ( $locations as $location => $menu_id ) {
		if ( $term_id == $menu_id ) {
			$locations[ $location ] = $new_term_id;
		}
	}
	set_theme_mod( 'nav_menu_locations', $locations );
}