Source

File: wp-admin/includes/class-wp-automatic-updater.php

protected function send_core_update_notification_email( $item ) {
		$notified = get_site_option( 'auto_core_update_notified' );

		// Don't notify if we've already notified the same email address of the same version.
		if ( $notified && get_site_option( 'admin_email' ) === $notified['email'] && $notified['version'] == $item->current ) {
			return false;
		}

		// See if we need to notify users of a core update.
		$notify = ! empty( $item->notify_email );

		/**
		 * Filters whether to notify the site administrator of a new core update.
		 *
		 * By default, administrators are notified when the update offer received
		 * from WordPress.org sets a particular flag. This allows some discretion
		 * in if and when to notify.
		 *
		 * This filter is only evaluated once per release. If the same email address
		 * was already notified of the same new version, WordPress won't repeatedly
		 * email the administrator.
		 *
		 * This filter is also used on about.php to check if a plugin has disabled
		 * these notifications.
		 *
		 * @since 3.7.0
		 *
		 * @param bool   $notify Whether the site administrator is notified.
		 * @param object $item   The update offer.
		 */
		if ( ! apply_filters( 'send_core_update_notification_email', $notify, $item ) ) {
			return false;
		}

		$this->send_email( 'manual', $item );
		return true;
	}