Testing mu-plugin for https://core.trac.wordpress.org/ticket/56351

an-unsplit-term.php

<?php

/**
 * Prevent the term splitting job from being fired.
 */
add_filter(
	'pre_schedule_event',
	function( $pre, $event ) {
		if ( $event->hook === 'wp_split_shared_term_batch' ) {
			return false;
		}

		return $pre;
	},
	10,
	2
);

if ( ! term_exists( 'pw-unsplit-term' ) ) {
	$wpdb->insert(
		$wpdb->terms,
		array(
			'name' => 'PW Unsplit term',
			'slug' => 'pw-unsplit-term',
		)
	);

	$term_id = $wpdb->insert_id;

	$wpdb->insert(
		$wpdb->term_taxonomy,
		array(
			'term_id'  => $term_id,
			'taxonomy' => 'post_tag',
			'parent'   => 0,
			'count'    => 0,
		)
	);

	$wpdb->insert(
		$wpdb->term_taxonomy,
		array(
			'term_id'  => $term_id,
			'taxonomy' => 'category',
			'parent'   => 0,
			'count'    => 0,
		)
	);
}