Source

File: wp-admin/includes/network.php

function allow_subdirectory_install() {
	global $wpdb;

	/**
	 * Filters whether to enable the subdirectory installation feature in Multisite.
	 *
	 * @since 3.0.0
	 *
	 * @param bool $allow Whether to enable the subdirectory installation feature in Multisite.
	 *                    Default false.
	 */
	if ( apply_filters( 'allow_subdirectory_install', false ) ) {
		return true;
	}

	if ( defined( 'ALLOW_SUBDIRECTORY_INSTALL' ) && ALLOW_SUBDIRECTORY_INSTALL ) {
		return true;
	}

	$post = $wpdb->get_row( "SELECT ID FROM $wpdb->posts WHERE post_date < DATE_SUB(NOW(), INTERVAL 1 MONTH) AND post_status = 'publish'" );
	if ( empty( $post ) ) {
		return true;
	}

	return false;
}