BookStack/app/Entities/Tools/Markdown/CustomStrikeThroughExtension.php

<?php namespace BookStack\Entities\Tools\Markdown;

use League\CommonMark\ConfigurableEnvironmentInterface;
use League\CommonMark\Extension\ExtensionInterface;
use League\CommonMark\Extension\Strikethrough\Strikethrough;
use League\CommonMark\Extension\Strikethrough\StrikethroughDelimiterProcessor;

class CustomStrikeThroughExtension implements ExtensionInterface
{

    public function register(ConfigurableEnvironmentInterface $environment)
    {
        $environment->addDelimiterProcessor(new StrikethroughDelimiterProcessor());
        $environment->addInlineRenderer(Strikethrough::class, new CustomStrikethroughRenderer());
    }
}