FilterListPlugin - https://magento.stackexchange.com/questions/111781/how-to-keep-all-filters-visible-in-magento-2

FilterListPlugin.php

<?php
namespace Demo\Navigation\Plugin\Layer;

use Magento\Catalog\Model\ResourceModel\Layer\Filter\Attribute;
use Magento\Framework\App\RequestInterface;
use Magento\Catalog\Model\Layer\Filter\ItemFactory;
use Magento\Catalog\Model\Category;
use Magento\Catalog\Model\Layer\FilterList;
use Magento\Catalog\Model\Product;

class FilterListPlugin
{
    /**
     * @var ItemFactory
     */
    protected $itemFactory;
    /**
     * @var Attribute
     */
    protected $attributeResource;
    /**
     * @var RequestInterface
     */
    protected $requestInterface;

    /**
     * FilterListPlugin constructor.
     * @param ItemFactory $itemFactory
     * @param Attribute $attributeResource
     * @param RequestInterface $requestInterface
     */
    public function __construct(
        ItemFactory $itemFactory,
        Attribute $attributeResource,
        RequestInterface $requestInterface
    ) {
        $this->itemFactory = $itemFactory;
        $this->attributeResource = $attributeResource;
        $this->requestInterface = $requestInterface;
    }

    /**
     * Only allow defined attributes as filters
     * and sort them in the needed order
     *
     * @param  FilterList $subject
     * @param  $result
     * @return array
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function afterGetFilters(FilterList $subject, $result) //@codingStandardsIgnoreLine
    {
        $data = [
            'cat' => Category::ENTITY,
            'demo_product_type' => Product::ENTITY,
            'demo_material' => Product::ENTITY,
            'demo_packaging' => Product::ENTITY,
            'demo_characteristics' => Product::ENTITY,
        ];

        foreach ($result as $filter) {
            if (in_array($filter->getRequestVar(), array_keys($data))) {
                if ($data[$filter->getRequestVar()] === Category::ENTITY) {
                    $filter->setItems(
                        $this->getCategoryItems($filter)
                    );
                }

                if ($data[$filter->getRequestVar()] === Product::ENTITY) {
                    $filter->setItems(
                        $this->getAttributeItems($filter)
                    );
                }

                $data[$filter->getRequestVar()] = $filter;
            }
        }

        return $data;
    }

    /**
     * @param $filter
     * @param $label
     * @param $value
     * @param int    $count
     * @param bool   $active
     * @return mixed
     */
    protected function _createItem($filter, $label, $value, $count = 0, $active = false)
    {
        return $this->itemFactory->create()
            ->setFilter($filter)
            ->setLabel($label)
            ->setValue($value)
            ->setCount($count)
            ->setIsActive($active);
    }

    /**
     * @param $filter
     * @return array
     */
    protected function getAttributeItems($filter)
    {
        $_data = [];
        $attribute = $filter->getAttributeModel();
        foreach ($attribute->getOptions() as $option) {
            if (empty($option['value']) || !$attribute->getIsFilterable()) {
                continue;
            }
            $options = $this->attributeResource->getCount($filter);
            $count = isset($options[$option->getvalue()]) ? $options[$option->getvalue()] : 0;
            if ($this->requestInterface->getParam($filter->getRequestVar()) == $option->getValue()) {
                $active = true;
            } else {
                $active = false;
            }
            $_data[] = $this->_createItem(
                $filter,
                $option->getLabel(),
                $option->getValue(),
                $count,
                $active
            );
        }
        return $_data;
    }

    /**
     * @param $filter
     * @return array
     */
    protected function getCategoryItems($filter)
    {
        $_data = [];

        $category = $filter->getLayer()->getCurrentCategory();
        if ($category->hasChildren()) {
            foreach ($category->getChildrenCategories() as $child) {
                if ($child->getProductCount() > 0 && $child->getIsActive()) {
                    if ($this->requestInterface->getParam($filter->getRequestVar())
                        && $this->requestInterface->getParam($filter->getRequestVar()) == $child->getId()
                    ) {
                        $active = true;
                    } else {
                        $active = false;
                    }

                    $_data[$child->getPosition()] = $this->_createItem(
                        $filter,
                        $child->getName(),
                        $child->getId(),
                        $child->getProductCount(),
                        $active
                    );
                }
            }
        }
        ksort($_data);
        return $_data;
    }
}

Hi I would really like to use this in 2.2
But how can i install this?

Where i add this?

This worked great on Magento 2.2.6!
For those of you wondering, you have to add this as a plugin to Magento\Catalog\Model\Layer\FilterList on your frontend di.xml.

hello,
I'm implementing this in Magento 2.3, But it is not working, Can you help me or Can you give me any idea