AddCustomCategoryDisplayMode

di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Catalog\Model\Category\Attribute\Source\Mode">
        <plugin name="magevision_blog38_category_attribute_source_mode" type="MageVision\Blog38\Plugin\Catalog\Model\Category\Attribute\Source\Mode"/>
    </type>
</config>

Mode.php

<?php
namespace MageVision\Blog38\Plugin\Catalog\Model\Category\Attribute\Source;

class Mode
{
    /**
     * @param \Magento\Catalog\Model\Category\Attribute\Source\Mode $subject
     * @param $result
     * @return array
     */
    public function afterGetAllOptions(
        \Magento\Catalog\Model\Category\Attribute\Source\Mode $subject,
        $result
    ) {
        $result[] = ['value' => 'CUSTOM_MODE', 'label' => 'Custom Mode'];
        return $result;
    }
}