This article is a solution If you want to add new custom sorter or filter options such as below:

New product,
Name, Instock,
Price Low to high,
Price High to Low.

Create a custom module and follow the below step.

1. Create file di.xml on app/code/Magemonkeys/Sorting/etc

<?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="MagentoCatalogModelConfig">
        <plugin name="Magemonkeys_Sorting::addCustomSortOptions" type="MagemonkeysSortingPluginModelConfig" />
    </type>
    <type name="MagentoCatalogBlockProductProductListToolbar">
        <plugin name="Magemonkeys_Sorting::implementCustomSortOptions" type="MagemonkeysSortingPluginProductProductListToolbar" />
    </type>
</config>

2. Create file Config.php on app/code/Magemonkeys/Sorting/Plugin

<?php
namespace MagemonkeysSortingPluginModel;

class Config
{    
    public function afterGetAttributeUsedForSortByArray(MagentoCatalogModelConfig $catalogConfig, $options)
    {
        //Remove default sorting options
        unset($options['position']);
        unset($options['name']);
        unset($options['price']);

        //New sorting options
        $options['newest'] = __('New product');        
        $options['is_salable'] = __('Stock');
        $options['name'] = __('Name');        
        $options['price_desc'] = __('Price High to Low');
        $options['price_asc'] = __('Price Low to High');

        return $options;
    }
}

3. Create file Toolbar.php on app/code/Magemonkeys/Sorting/Product/ProductList

<?php
namespace MagemonkeysSortingPluginProductProductList;

class Toolbar
{    
    public function aroundSetCollection(
        MagentoCatalogBlockProductProductListToolbar $toolbar,
        Closure $proceed,
        $collection
    ) {
        $this->_collection = $collection;
        $currentOrder = $toolbar->getCurrentOrder();
        $currentDirection = $toolbar->getCurrentDirection();
        $result = $proceed($collection);

        if ($currentOrder) {
            switch ($currentOrder) {

            case 'newest':
                if ($currentDirection == 'desc') {
                    $this->_collection->getSelect()->order('e.created_at ASC');
                } elseif ($currentDirection == 'asc') {
                    $this->_collection->getSelect()->order('e.created_at DESC');
                }
            break;

            case 'is_salable': 
                if ($currentDirection == 'desc') {
                    $this->_collection->getSelect()->order('qty ASC');
                } elseif ($currentDirection == 'asc') {
                    $this->_collection->getSelect()->order('qty DESC');
                }               
                          
            break;            

            case 'name':
                if ($currentDirection == 'desc') {
                    $this->_collection->getSelect()->order('name ASC');
                } elseif ($currentDirection == 'asc') {
                    $this->_collection->getSelect()->order('name DESC');
                }
            break;

            case 'price_desc':            
                if ($currentDirection == 'desc') {
                    $this->_collection->getSelect()->order('price ASC');
                } elseif ($currentDirection == 'asc') {
                    $this->_collection->getSelect()->order('price DESC');
                }
            break;

            case 'price_asc':
                if ($currentDirection == 'desc') {
                    $this->_collection->getSelect()->order('price DESC');
                } elseif ($currentDirection == 'asc') {
                    $this->_collection->getSelect()->order('price ASC');
                }
            break;            

            default:        
                $this->_collection
                    ->setOrder($currentOrder, $currentDirection);
            break;

            }
        }        
        //var_dump($currentDirection);
        return $result;
    }
}