wp_get_split_terms() │ WP 4.2.0

The recommended fixing method is the split_shared_term hook. However, when the plugin is updated after splitting, it is no longer possible to stick to this hook. In this case wp_get_split_terms(), which stores split information, will help.

Suppose the plugin stored term IDs in the featured_tags option. We need to make sure that none of the terms stored in featured_tags have been split, and if so, we need to update the IDs in that option.

function featured_tags_check_for_split_terms() {

	$featured_tag_ids = get_option( 'featured_tags', array() );

	// Check if there are ID terms from the taxonomy `post_tag` that have been separated
	foreach ( $featured_tag_ids as $index => $featured_tag_id ) {

		$split_terms = wp_get_split_terms( $featured_tag_id, 'post_tag' );

		if ( ! empty( $split_terms ) ) {
			foreach ( $split_terms as $split_term ) {
				// Replacing old IDs with new ones
				$featured_tag_ids[ $index ] = $split_term['post_tag'];
			}
		}
	}

	// Update
	update_option( 'featured_tags', $featured_tag_ids );
}

To check if the taxonomy elements have been separated at all, run this code:

$split_terms = get_option( '_split_terms', [] );

if( $split_terms ){
	// разделение было и есть данные о разделении
}
else {
	// данных о разделении нет (но код разделения скорее всего сработал)
	// просто у вас не оказалось терминов с одинаковыми ИД
}

print_r( $split_terms );

/* displays:
Array
(
	[3] => Array
		(
			[post_tag] => 591
		)

	[18] => Array
		(
			[post_tag] => 592
		)
)
*/

If array() prints out the result, you did not have terms with the same ID after the split and the option can be removed without consequences:

delete_option( '_split_terms' );